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7 <chapter id="drd-manual" xreflabel="DRD: a thread error detector">
8 <title>DRD: a thread error detector</title>
10 <para>To use this tool, you must specify
11 <option>--tool=drd</option>
12 on the Valgrind command line.</para>
15 <sect1 id="drd-manual.overview" xreflabel="Overview">
16 <title>Overview</title>
18 <para>
19 DRD is a Valgrind tool for detecting errors in multithreaded C and C++
20 programs. The tool works for any program that uses the POSIX threading
21 primitives or that uses threading concepts built on top of the POSIX threading
22 primitives.
23 </para>
25 <sect2 id="drd-manual.mt-progr-models" xreflabel="MT-progr-models">
26 <title>Multithreaded Programming Paradigms</title>
28 <para>
29 There are two possible reasons for using multithreading in a program:
30 <itemizedlist>
31 <listitem>
32 <para>
33 To model concurrent activities. Assigning one thread to each activity
34 can be a great simplification compared to multiplexing the states of
35 multiple activities in a single thread. This is why most server software
36 and embedded software is multithreaded.
37 </para>
38 </listitem>
39 <listitem>
40 <para>
41 To use multiple CPU cores simultaneously for speeding up
42 computations. This is why many High Performance Computing (HPC)
43 applications are multithreaded.
44 </para>
45 </listitem>
46 </itemizedlist>
47 </para>
49 <para>
50 Multithreaded programs can use one or more of the following programming
51 paradigms. Which paradigm is appropriate depends e.g. on the application type.
52 Some examples of multithreaded programming paradigms are:
53 <itemizedlist>
54 <listitem>
55 <para>
56 Locking. Data that is shared over threads is protected from concurrent
57 accesses via locking. E.g. the POSIX threads library, the Qt library
58 and the Boost.Thread library support this paradigm directly.
59 </para>
60 </listitem>
61 <listitem>
62 <para>
63 Message passing. No data is shared between threads, but threads exchange
64 data by passing messages to each other. Examples of implementations of
65 the message passing paradigm are MPI and CORBA.
66 </para>
67 </listitem>
68 <listitem>
69 <para>
70 Automatic parallelization. A compiler converts a sequential program into
71 a multithreaded program. The original program may or may not contain
72 parallelization hints. One example of such parallelization hints is the
73 OpenMP standard. In this standard a set of directives are defined which
74 tell a compiler how to parallelize a C, C++ or Fortran program. OpenMP
75 is well suited for computational intensive applications. As an example,
76 an open source image processing software package is using OpenMP to
77 maximize performance on systems with multiple CPU
78 cores. GCC supports the
79 OpenMP standard from version 4.2.0 on.
80 </para>
81 </listitem>
82 <listitem>
83 <para>
84 Software Transactional Memory (STM). Any data that is shared between
85 threads is updated via transactions. After each transaction it is
86 verified whether there were any conflicting transactions. If there were
87 conflicts, the transaction is aborted, otherwise it is committed. This
88 is a so-called optimistic approach. There is a prototype of the Intel C++
89 Compiler available that supports STM. Research about the addition of
90 STM support to GCC is ongoing.
91 </para>
92 </listitem>
93 </itemizedlist>
94 </para>
96 <para>
97 DRD supports any combination of multithreaded programming paradigms as
98 long as the implementation of these paradigms is based on the POSIX
99 threads primitives. DRD however does not support programs that use
100 e.g. Linux' futexes directly. Attempts to analyze such programs with
101 DRD will cause DRD to report many false positives.
102 </para>
104 </sect2>
107 <sect2 id="drd-manual.pthreads-model" xreflabel="Pthreads-model">
108 <title>POSIX Threads Programming Model</title>
110 <para>
111 POSIX threads, also known as Pthreads, is the most widely available
112 threading library on Unix systems.
113 </para>
115 <para>
116 The POSIX threads programming model is based on the following abstractions:
117 <itemizedlist>
118 <listitem>
119 <para>
120 A shared address space. All threads running within the same
121 process share the same address space. All data, whether shared or
122 not, is identified by its address.
123 </para>
124 </listitem>
125 <listitem>
126 <para>
127 Regular load and store operations, which allow to read values
128 from or to write values to the memory shared by all threads
129 running in the same process.
130 </para>
131 </listitem>
132 <listitem>
133 <para>
134 Atomic store and load-modify-store operations. While these are
135 not mentioned in the POSIX threads standard, most
136 microprocessors support atomic memory operations.
137 </para>
138 </listitem>
139 <listitem>
140 <para>
141 Threads. Each thread represents a concurrent activity.
142 </para>
143 </listitem>
144 <listitem>
145 <para>
146 Synchronization objects and operations on these synchronization
147 objects. The following types of synchronization objects have been
148 defined in the POSIX threads standard: mutexes, condition variables,
149 semaphores, reader-writer synchronization objects, barriers and
150 spinlocks.
151 </para>
152 </listitem>
153 </itemizedlist>
154 </para>
156 <para>
157 Which source code statements generate which memory accesses depends on
158 the <emphasis>memory model</emphasis> of the programming language being
159 used. There is not yet a definitive memory model for the C and C++
160 languages. For a draft memory model, see also the document
161 <ulink url="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2338.html">
162 WG21/N2338: Concurrency memory model compiler consequences</ulink>.
163 </para>
165 <para>
166 For more information about POSIX threads, see also the Single UNIX
167 Specification version 3, also known as
168 <ulink url="http://www.opengroup.org/onlinepubs/000095399/idx/threads.html">
169 IEEE Std 1003.1</ulink>.
170 </para>
172 </sect2>
175 <sect2 id="drd-manual.mt-problems" xreflabel="MT-Problems">
176 <title>Multithreaded Programming Problems</title>
178 <para>
179 Depending on which multithreading paradigm is being used in a program,
180 one or more of the following problems can occur:
181 <itemizedlist>
182 <listitem>
183 <para>
184 Data races. One or more threads access the same memory location without
185 sufficient locking. Most but not all data races are programming errors
186 and are the cause of subtle and hard-to-find bugs.
187 </para>
188 </listitem>
189 <listitem>
190 <para>
191 Lock contention. One thread blocks the progress of one or more other
192 threads by holding a lock too long.
193 </para>
194 </listitem>
195 <listitem>
196 <para>
197 Improper use of the POSIX threads API. Most implementations of the POSIX
198 threads API have been optimized for runtime speed. Such implementations
199 will not complain on certain errors, e.g. when a mutex is being unlocked
200 by another thread than the thread that obtained a lock on the mutex.
201 </para>
202 </listitem>
203 <listitem>
204 <para>
205 Deadlock. A deadlock occurs when two or more threads wait for
206 each other indefinitely.
207 </para>
208 </listitem>
209 <listitem>
210 <para>
211 False sharing. If threads that run on different processor cores
212 access different variables located in the same cache line
213 frequently, this will slow down the involved threads a lot due
214 to frequent exchange of cache lines.
215 </para>
216 </listitem>
217 </itemizedlist>
218 </para>
220 <para>
221 Although the likelihood of the occurrence of data races can be reduced
222 through a disciplined programming style, a tool for automatic
223 detection of data races is a necessity when developing multithreaded
224 software. DRD can detect these, as well as lock contention and
225 improper use of the POSIX threads API.
226 </para>
228 </sect2>
231 <sect2 id="drd-manual.data-race-detection" xreflabel="data-race-detection">
232 <title>Data Race Detection</title>
234 <para>
235 The result of load and store operations performed by a multithreaded program
236 depends on the order in which memory operations are performed. This order is
237 determined by:
238 <orderedlist>
239 <listitem>
240 <para>
241 All memory operations performed by the same thread are performed in
242 <emphasis>program order</emphasis>, that is, the order determined by the
243 program source code and the results of previous load operations.
244 </para>
245 </listitem>
246 <listitem>
247 <para>
248 Synchronization operations determine certain ordering constraints on
249 memory operations performed by different threads. These ordering
250 constraints are called the <emphasis>synchronization order</emphasis>.
251 </para>
252 </listitem>
253 </orderedlist>
254 The combination of program order and synchronization order is called the
255 <emphasis>happens-before relationship</emphasis>. This concept was first
256 defined by S. Adve et al in the paper <emphasis>Detecting data races on weak
257 memory systems</emphasis>, ACM SIGARCH Computer Architecture News, v.19 n.3,
258 p.234-243, May 1991.
259 </para>
261 <para>
262 Two memory operations <emphasis>conflict</emphasis> if both operations are
263 performed by different threads, refer to the same memory location and at least
264 one of them is a store operation.
265 </para>
267 <para>
268 A multithreaded program is <emphasis>data-race free</emphasis> if all
269 conflicting memory accesses are ordered by synchronization
270 operations.
271 </para>
273 <para>
274 A well known way to ensure that a multithreaded program is data-race
275 free is to ensure that a locking discipline is followed. It is e.g.
276 possible to associate a mutex with each shared data item, and to hold
277 a lock on the associated mutex while the shared data is accessed.
278 </para>
280 <para>
281 All programs that follow a locking discipline are data-race free, but not all
282 data-race free programs follow a locking discipline. There exist multithreaded
283 programs where access to shared data is arbitrated via condition variables,
284 semaphores or barriers. As an example, a certain class of HPC applications
285 consists of a sequence of computation steps separated in time by barriers, and
286 where these barriers are the only means of synchronization. Although there are
287 many conflicting memory accesses in such applications and although such
288 applications do not make use mutexes, most of these applications do not
289 contain data races.
290 </para>
292 <para>
293 There exist two different approaches for verifying the correctness of
294 multithreaded programs at runtime. The approach of the so-called Eraser
295 algorithm is to verify whether all shared memory accesses follow a consistent
296 locking strategy. And the happens-before data race detectors verify directly
297 whether all interthread memory accesses are ordered by synchronization
298 operations. While the last approach is more complex to implement, and while it
299 is more sensitive to OS scheduling, it is a general approach that works for
300 all classes of multithreaded programs. An important advantage of
301 happens-before data race detectors is that these do not report any false
302 positives.
303 </para>
305 <para>
306 DRD is based on the happens-before algorithm.
307 </para>
309 </sect2>
312 </sect1>
315 <sect1 id="drd-manual.using-drd" xreflabel="Using DRD">
316 <title>Using DRD</title>
318 <sect2 id="drd-manual.options" xreflabel="DRD Command-line Options">
319 <title>DRD Command-line Options</title>
321 <para>The following command-line options are available for controlling the
322 behavior of the DRD tool itself:</para>
324 <!-- start of xi:include in the manpage -->
325 <variablelist id="drd.opts.list">
326 <varlistentry>
327 <term>
328 <option><![CDATA[--check-stack-var=<yes|no> [default: no]]]></option>
329 </term>
330 <listitem>
331 <para>
332 Controls whether DRD detects data races on stack
333 variables. Verifying stack variables is disabled by default because
334 most programs do not share stack variables over threads.
335 </para>
336 </listitem>
337 </varlistentry>
338 <varlistentry>
339 <term>
340 <option><![CDATA[--exclusive-threshold=<n> [default: off]]]></option>
341 </term>
342 <listitem>
343 <para>
344 Print an error message if any mutex or writer lock has been
345 held longer than the time specified in milliseconds. This
346 option enables the detection of lock contention.
347 </para>
348 </listitem>
349 </varlistentry>
350 <varlistentry>
351 <term>
352 <option><![CDATA[--join-list-vol=<n> [default: 10]]]></option>
353 </term>
354 <listitem>
355 <para>
356 Data races that occur between a statement at the end of one thread
357 and another thread can be missed if memory access information is
358 discarded immediately after a thread has been joined. This option
359 allows to specify for how many joined threads memory access information
360 should be retained.
361 </para>
362 </listitem>
363 </varlistentry>
364 <varlistentry>
365 <term>
366 <option>
367 <![CDATA[--first-race-only=<yes|no> [default: no]]]>
368 </option>
369 </term>
370 <listitem>
371 <para>
372 Whether to report only the first data race that has been detected on a
373 memory location or all data races that have been detected on a memory
374 location.
375 </para>
376 </listitem>
377 </varlistentry>
378 <varlistentry>
379 <term>
380 <option>
381 <![CDATA[--free-is-write=<yes|no> [default: no]]]>
382 </option>
383 </term>
384 <listitem>
385 <para>
386 Whether to report races between accessing memory and freeing
387 memory. Enabling this option may cause DRD to run slightly
388 slower. Notes:</para>
389 <itemizedlist>
390 <listitem>
391 <para>
392 Don't enable this option when using custom memory allocators
393 that use
394 the <computeroutput>VG_USERREQ__MALLOCLIKE_BLOCK</computeroutput>
395 and <computeroutput>VG_USERREQ__FREELIKE_BLOCK</computeroutput>
396 because that would result in false positives.
397 </para>
398 </listitem>
399 <listitem>
400 <para>Don't enable this option when using reference-counted
401 objects because that will result in false positives, even when
402 that code has been annotated properly with
403 <computeroutput>ANNOTATE_HAPPENS_BEFORE</computeroutput>
404 and <computeroutput>ANNOTATE_HAPPENS_AFTER</computeroutput>. See
405 e.g. the output of the following command for an example:
406 <computeroutput>valgrind --tool=drd --free-is-write=yes
407 drd/tests/annotate_smart_pointer</computeroutput>.
408 </para>
409 </listitem>
410 </itemizedlist>
411 </listitem>
412 </varlistentry>
413 <varlistentry>
414 <term>
415 <option>
416 <![CDATA[--report-signal-unlocked=<yes|no> [default: yes]]]>
417 </option>
418 </term>
419 <listitem>
420 <para>
421 Whether to report calls to
422 <function>pthread_cond_signal</function> and
423 <function>pthread_cond_broadcast</function> where the mutex
424 associated with the signal through
425 <function>pthread_cond_wait</function> or
426 <function>pthread_cond_timed_wait</function>is not locked at
427 the time the signal is sent. Sending a signal without holding
428 a lock on the associated mutex is a common programming error
429 which can cause subtle race conditions and unpredictable
430 behavior. There exist some uncommon synchronization patterns
431 however where it is safe to send a signal without holding a
432 lock on the associated mutex.
433 </para>
434 </listitem>
435 </varlistentry>
436 <varlistentry>
437 <term>
438 <option><![CDATA[--segment-merging=<yes|no> [default: yes]]]></option>
439 </term>
440 <listitem>
441 <para>
442 Controls segment merging. Segment merging is an algorithm to
443 limit memory usage of the data race detection
444 algorithm. Disabling segment merging may improve the accuracy
445 of the so-called 'other segments' displayed in race reports
446 but can also trigger an out of memory error.
447 </para>
448 </listitem>
449 </varlistentry>
450 <varlistentry>
451 <term>
452 <option><![CDATA[--segment-merging-interval=<n> [default: 10]]]></option>
453 </term>
454 <listitem>
455 <para>
456 Perform segment merging only after the specified number of new
457 segments have been created. This is an advanced configuration option
458 that allows to choose whether to minimize DRD's memory usage by
459 choosing a low value or to let DRD run faster by choosing a slightly
460 higher value. The optimal value for this parameter depends on the
461 program being analyzed. The default value works well for most programs.
462 </para>
463 </listitem>
464 </varlistentry>
465 <varlistentry>
466 <term>
467 <option><![CDATA[--shared-threshold=<n> [default: off]]]></option>
468 </term>
469 <listitem>
470 <para>
471 Print an error message if a reader lock has been held longer
472 than the specified time (in milliseconds). This option enables
473 the detection of lock contention.
474 </para>
475 </listitem>
476 </varlistentry>
477 <varlistentry>
478 <term>
479 <option><![CDATA[--show-confl-seg=<yes|no> [default: yes]]]></option>
480 </term>
481 <listitem>
482 <para>
483 Show conflicting segments in race reports. Since this
484 information can help to find the cause of a data race, this
485 option is enabled by default. Disabling this option makes the
486 output of DRD more compact.
487 </para>
488 </listitem>
489 </varlistentry>
490 <varlistentry>
491 <term>
492 <option><![CDATA[--show-stack-usage=<yes|no> [default: no]]]></option>
493 </term>
494 <listitem>
495 <para>
496 Print stack usage at thread exit time. When a program creates a large
497 number of threads it becomes important to limit the amount of virtual
498 memory allocated for thread stacks. This option makes it possible to
499 observe how much stack memory has been used by each thread of the
500 client program. Note: the DRD tool itself allocates some temporary
501 data on the client thread stack. The space necessary for this
502 temporary data must be allocated by the client program when it
503 allocates stack memory, but is not included in stack usage reported by
504 DRD.
505 </para>
506 </listitem>
507 </varlistentry>
508 </variablelist>
509 <!-- end of xi:include in the manpage -->
511 <!-- start of xi:include in the manpage -->
512 <para>
513 The following options are available for monitoring the behavior of the
514 client program:
515 </para>
517 <variablelist id="drd.debugopts.list">
518 <varlistentry>
519 <term>
520 <option><![CDATA[--trace-addr=<address> [default: none]]]></option>
521 </term>
522 <listitem>
523 <para>
524 Trace all load and store activity for the specified
525 address. This option may be specified more than once.
526 </para>
527 </listitem>
528 </varlistentry>
529 <varlistentry>
530 <term>
531 <option><![CDATA[--ptrace-addr=<address> [default: none]]]></option>
532 </term>
533 <listitem>
534 <para>
535 Trace all load and store activity for the specified address and keep
536 doing that even after the memory at that address has been freed and
537 reallocated.
538 </para>
539 </listitem>
540 </varlistentry>
541 <varlistentry>
542 <term>
543 <option><![CDATA[--trace-alloc=<yes|no> [default: no]]]></option>
544 </term>
545 <listitem>
546 <para>
547 Trace all memory allocations and deallocations. May produce a huge
548 amount of output.
549 </para>
550 </listitem>
551 </varlistentry>
552 <varlistentry>
553 <term>
554 <option><![CDATA[--trace-barrier=<yes|no> [default: no]]]></option>
555 </term>
556 <listitem>
557 <para>
558 Trace all barrier activity.
559 </para>
560 </listitem>
561 </varlistentry>
562 <varlistentry>
563 <term>
564 <option><![CDATA[--trace-cond=<yes|no> [default: no]]]></option>
565 </term>
566 <listitem>
567 <para>
568 Trace all condition variable activity.
569 </para>
570 </listitem>
571 </varlistentry>
572 <varlistentry>
573 <term>
574 <option><![CDATA[--trace-fork-join=<yes|no> [default: no]]]></option>
575 </term>
576 <listitem>
577 <para>
578 Trace all thread creation and all thread termination events.
579 </para>
580 </listitem>
581 </varlistentry>
582 <varlistentry>
583 <term>
584 <option><![CDATA[--trace-hb=<yes|no> [default: no]]]></option>
585 </term>
586 <listitem>
587 <para>
588 Trace execution of the <literal>ANNOTATE_HAPPENS_BEFORE()</literal>,
589 <literal>ANNOTATE_HAPPENS_AFTER()</literal> and
590 <literal>ANNOTATE_HAPPENS_DONE()</literal> client requests.
591 </para>
592 </listitem>
593 </varlistentry>
594 <varlistentry>
595 <term>
596 <option><![CDATA[--trace-mutex=<yes|no> [default: no]]]></option>
597 </term>
598 <listitem>
599 <para>
600 Trace all mutex activity.
601 </para>
602 </listitem>
603 </varlistentry>
604 <varlistentry>
605 <term>
606 <option><![CDATA[--trace-rwlock=<yes|no> [default: no]]]></option>
607 </term>
608 <listitem>
609 <para>
610 Trace all reader-writer lock activity.
611 </para>
612 </listitem>
613 </varlistentry>
614 <varlistentry>
615 <term>
616 <option><![CDATA[--trace-semaphore=<yes|no> [default: no]]]></option>
617 </term>
618 <listitem>
619 <para>
620 Trace all semaphore activity.
621 </para>
622 </listitem>
623 </varlistentry>
624 </variablelist>
625 <!-- end of xi:include in the manpage -->
627 </sect2>
630 <sect2 id="drd-manual.data-races" xreflabel="Data Races">
631 <title>Detected Errors: Data Races</title>
633 <para>
634 DRD prints a message every time it detects a data race. Please keep
635 the following in mind when interpreting DRD's output:
636 <itemizedlist>
637 <listitem>
638 <para>
639 Every thread is assigned a <emphasis>thread ID</emphasis> by the DRD
640 tool. A thread ID is a number. Thread ID's start at one and are never
641 recycled.
642 </para>
643 </listitem>
644 <listitem>
645 <para>
646 The term <emphasis>segment</emphasis> refers to a consecutive
647 sequence of load, store and synchronization operations, all
648 issued by the same thread. A segment always starts and ends at a
649 synchronization operation. Data race analysis is performed
650 between segments instead of between individual load and store
651 operations because of performance reasons.
652 </para>
653 </listitem>
654 <listitem>
655 <para>
656 There are always at least two memory accesses involved in a data
657 race. Memory accesses involved in a data race are called
658 <emphasis>conflicting memory accesses</emphasis>. DRD prints a
659 report for each memory access that conflicts with a past memory
660 access.
661 </para>
662 </listitem>
663 </itemizedlist>
664 </para>
666 <para>
667 Below you can find an example of a message printed by DRD when it
668 detects a data race:
669 </para>
670 <programlisting><![CDATA[
671 $ valgrind --tool=drd --read-var-info=yes drd/tests/rwlock_race
672 ...
673 ==9466== Thread 3:
674 ==9466== Conflicting load by thread 3 at 0x006020b8 size 4
675 ==9466== at 0x400B6C: thread_func (rwlock_race.c:29)
676 ==9466== by 0x4C291DF: vg_thread_wrapper (drd_pthread_intercepts.c:186)
677 ==9466== by 0x4E3403F: start_thread (in /lib64/libpthread-2.8.so)
678 ==9466== by 0x53250CC: clone (in /lib64/libc-2.8.so)
679 ==9466== Location 0x6020b8 is 0 bytes inside local var "s_racy"
680 ==9466== declared at rwlock_race.c:18, in frame #0 of thread 3
681 ==9466== Other segment start (thread 2)
682 ==9466== at 0x4C2847D: pthread_rwlock_rdlock* (drd_pthread_intercepts.c:813)
683 ==9466== by 0x400B6B: thread_func (rwlock_race.c:28)
684 ==9466== by 0x4C291DF: vg_thread_wrapper (drd_pthread_intercepts.c:186)
685 ==9466== by 0x4E3403F: start_thread (in /lib64/libpthread-2.8.so)
686 ==9466== by 0x53250CC: clone (in /lib64/libc-2.8.so)
687 ==9466== Other segment end (thread 2)
688 ==9466== at 0x4C28B54: pthread_rwlock_unlock* (drd_pthread_intercepts.c:912)
689 ==9466== by 0x400B84: thread_func (rwlock_race.c:30)
690 ==9466== by 0x4C291DF: vg_thread_wrapper (drd_pthread_intercepts.c:186)
691 ==9466== by 0x4E3403F: start_thread (in /lib64/libpthread-2.8.so)
692 ==9466== by 0x53250CC: clone (in /lib64/libc-2.8.so)
693 ...
694 ]]></programlisting>
696 <para>
697 The above report has the following meaning:
698 <itemizedlist>
699 <listitem>
700 <para>
701 The number in the column on the left is the process ID of the
702 process being analyzed by DRD.
703 </para>
704 </listitem>
705 <listitem>
706 <para>
707 The first line ("Thread 3") tells you the thread ID for
708 the thread in which context the data race has been detected.
709 </para>
710 </listitem>
711 <listitem>
712 <para>
713 The next line tells which kind of operation was performed (load or
714 store) and by which thread. On the same line the start address and the
715 number of bytes involved in the conflicting access are also displayed.
716 </para>
717 </listitem>
718 <listitem>
719 <para>
720 Next, the call stack of the conflicting access is displayed. If
721 your program has been compiled with debug information
722 (<option>-g</option>), this call stack will include file names and
723 line numbers. The two
724 bottommost frames in this call stack (<function>clone</function>
725 and <function>start_thread</function>) show how the NPTL starts
726 a thread. The third frame
727 (<function>vg_thread_wrapper</function>) is added by DRD. The
728 fourth frame (<function>thread_func</function>) is the first
729 interesting line because it shows the thread entry point, that
730 is the function that has been passed as the third argument to
731 <function>pthread_create</function>.
732 </para>
733 </listitem>
734 <listitem>
735 <para>
736 Next, the allocation context for the conflicting address is
737 displayed. For dynamically allocated data the allocation call
738 stack is shown. For static variables and stack variables the
739 allocation context is only shown when the option
740 <option>--read-var-info=yes</option> has been
741 specified. Otherwise DRD will print <computeroutput>Allocation
742 context: unknown</computeroutput>.
743 </para>
744 </listitem>
745 <listitem>
746 <para>
747 A conflicting access involves at least two memory accesses. For
748 one of these accesses an exact call stack is displayed, and for
749 the other accesses an approximate call stack is displayed,
750 namely the start and the end of the segments of the other
751 accesses. This information can be interpreted as follows:
752 <orderedlist>
753 <listitem>
754 <para>
755 Start at the bottom of both call stacks, and count the
756 number stack frames with identical function name, file
757 name and line number. In the above example the three
758 bottommost frames are identical
759 (<function>clone</function>,
760 <function>start_thread</function> and
761 <function>vg_thread_wrapper</function>).
762 </para>
763 </listitem>
764 <listitem>
765 <para>
766 The next higher stack frame in both call stacks now tells
767 you between in which source code region the other memory
768 access happened. The above output tells that the other
769 memory access involved in the data race happened between
770 source code lines 28 and 30 in file
771 <computeroutput>rwlock_race.c</computeroutput>.
772 </para>
773 </listitem>
774 </orderedlist>
775 </para>
776 </listitem>
777 </itemizedlist>
778 </para>
780 </sect2>
783 <sect2 id="drd-manual.lock-contention" xreflabel="Lock Contention">
784 <title>Detected Errors: Lock Contention</title>
786 <para>
787 Threads must be able to make progress without being blocked for too long by
788 other threads. Sometimes a thread has to wait until a mutex or reader-writer
789 synchronization object is unlocked by another thread. This is called
790 <emphasis>lock contention</emphasis>.
791 </para>
793 <para>
794 Lock contention causes delays. Such delays should be as short as
795 possible. The two command line options
796 <literal>--exclusive-threshold=<n></literal> and
797 <literal>--shared-threshold=<n></literal> make it possible to
798 detect excessive lock contention by making DRD report any lock that
799 has been held longer than the specified threshold. An example:
800 </para>
801 <programlisting><![CDATA[
802 $ valgrind --tool=drd --exclusive-threshold=10 drd/tests/hold_lock -i 500
803 ...
804 ==10668== Acquired at:
805 ==10668== at 0x4C267C8: pthread_mutex_lock (drd_pthread_intercepts.c:395)
806 ==10668== by 0x400D92: main (hold_lock.c:51)
807 ==10668== Lock on mutex 0x7fefffd50 was held during 503 ms (threshold: 10 ms).
808 ==10668== at 0x4C26ADA: pthread_mutex_unlock (drd_pthread_intercepts.c:441)
809 ==10668== by 0x400DB5: main (hold_lock.c:55)
810 ...
811 ]]></programlisting>
813 <para>
814 The <literal>hold_lock</literal> test program holds a lock as long as
815 specified by the <literal>-i</literal> (interval) argument. The DRD
816 output reports that the lock acquired at line 51 in source file
817 <literal>hold_lock.c</literal> and released at line 55 was held during
818 503 ms, while a threshold of 10 ms was specified to DRD.
819 </para>
821 </sect2>
824 <sect2 id="drd-manual.api-checks" xreflabel="API Checks">
825 <title>Detected Errors: Misuse of the POSIX threads API</title>
827 <para>
828 DRD is able to detect and report the following misuses of the POSIX
829 threads API:
830 <itemizedlist>
831 <listitem>
832 <para>
833 Passing the address of one type of synchronization object
834 (e.g. a mutex) to a POSIX API call that expects a pointer to
835 another type of synchronization object (e.g. a condition
836 variable).
837 </para>
838 </listitem>
839 <listitem>
840 <para>
841 Attempts to unlock a mutex that has not been locked.
842 </para>
843 </listitem>
844 <listitem>
845 <para>
846 Attempts to unlock a mutex that was locked by another thread.
847 </para>
848 </listitem>
849 <listitem>
850 <para>
851 Attempts to lock a mutex of type
852 <literal>PTHREAD_MUTEX_NORMAL</literal> or a spinlock
853 recursively.
854 </para>
855 </listitem>
856 <listitem>
857 <para>
858 Destruction or deallocation of a locked mutex.
859 </para>
860 </listitem>
861 <listitem>
862 <para>
863 Sending a signal to a condition variable while no lock is held
864 on the mutex associated with the condition variable.
865 </para>
866 </listitem>
867 <listitem>
868 <para>
869 Calling <function>pthread_cond_wait</function> on a mutex
870 that is not locked, that is locked by another thread or that
871 has been locked recursively.
872 </para>
873 </listitem>
874 <listitem>
875 <para>
876 Associating two different mutexes with a condition variable
877 through <function>pthread_cond_wait</function>.
878 </para>
879 </listitem>
880 <listitem>
881 <para>
882 Destruction or deallocation of a condition variable that is
883 being waited upon.
884 </para>
885 </listitem>
886 <listitem>
887 <para>
888 Destruction or deallocation of a locked reader-writer synchronization
889 object.
890 </para>
891 </listitem>
892 <listitem>
893 <para>
894 Attempts to unlock a reader-writer synchronization object that was not
895 locked by the calling thread.
896 </para>
897 </listitem>
898 <listitem>
899 <para>
900 Attempts to recursively lock a reader-writer synchronization object
901 exclusively.
902 </para>
903 </listitem>
904 <listitem>
905 <para>
906 Attempts to pass the address of a user-defined reader-writer
907 synchronization object to a POSIX threads function.
908 </para>
909 </listitem>
910 <listitem>
911 <para>
912 Attempts to pass the address of a POSIX reader-writer synchronization
913 object to one of the annotations for user-defined reader-writer
914 synchronization objects.
915 </para>
916 </listitem>
917 <listitem>
918 <para>
919 Reinitialization of a mutex, condition variable, reader-writer
920 lock, semaphore or barrier.
921 </para>
922 </listitem>
923 <listitem>
924 <para>
925 Destruction or deallocation of a semaphore or barrier that is
926 being waited upon.
927 </para>
928 </listitem>
929 <listitem>
930 <para>
931 Missing synchronization between barrier wait and barrier destruction.
932 </para>
933 </listitem>
934 <listitem>
935 <para>
936 Exiting a thread without first unlocking the spinlocks, mutexes or
937 reader-writer synchronization objects that were locked by that thread.
938 </para>
939 </listitem>
940 <listitem>
941 <para>
942 Passing an invalid thread ID to <function>pthread_join</function>
943 or <function>pthread_cancel</function>.
944 </para>
945 </listitem>
946 </itemizedlist>
947 </para>
949 </sect2>
952 <sect2 id="drd-manual.clientreqs" xreflabel="Client requests">
953 <title>Client Requests</title>
955 <para>
956 Just as for other Valgrind tools it is possible to let a client program
957 interact with the DRD tool through client requests. In addition to the
958 client requests several macros have been defined that allow to use the
959 client requests in a convenient way.
960 </para>
962 <para>
963 The interface between client programs and the DRD tool is defined in
964 the header file <literal><valgrind/drd.h></literal>. The
965 available macros and client requests are:
966 <itemizedlist>
967 <listitem>
968 <para>
969 The macro <literal>DRD_GET_VALGRIND_THREADID</literal> and the
970 corresponding client
971 request <varname>VG_USERREQ__DRD_GET_VALGRIND_THREAD_ID</varname>.
972 Query the thread ID that has been assigned by the Valgrind core to the
973 thread executing this client request. Valgrind's thread ID's start at
974 one and are recycled in case a thread stops.
975 </para>
976 </listitem>
977 <listitem>
978 <para>
979 The macro <literal>DRD_GET_DRD_THREADID</literal> and the corresponding
980 client request <varname>VG_USERREQ__DRD_GET_DRD_THREAD_ID</varname>.
981 Query the thread ID that has been assigned by DRD to the thread
982 executing this client request. These are the thread ID's reported by DRD
983 in data race reports and in trace messages. DRD's thread ID's start at
984 one and are never recycled.
985 </para>
986 </listitem>
987 <listitem>
988 <para>
989 The macros <literal>DRD_IGNORE_VAR(x)</literal>,
990 <literal>ANNOTATE_TRACE_MEMORY(&x)</literal> and the corresponding
991 client request <varname>VG_USERREQ__DRD_START_SUPPRESSION</varname>. Some
992 applications contain intentional races. There exist e.g. applications
993 where the same value is assigned to a shared variable from two different
994 threads. It may be more convenient to suppress such races than to solve
995 these. This client request allows to suppress such races.
996 </para>
997 </listitem>
998 <listitem>
999 <para>
1000 The macro <literal>DRD_STOP_IGNORING_VAR(x)</literal> and the
1001 corresponding client request
1002 <varname>VG_USERREQ__DRD_FINISH_SUPPRESSION</varname>. Tell DRD
1003 to no longer ignore data races for the address range that was suppressed
1004 either via the macro <literal>DRD_IGNORE_VAR(x)</literal> or via the
1005 client request <varname>VG_USERREQ__DRD_START_SUPPRESSION</varname>.
1006 </para>
1007 </listitem>
1008 <listitem>
1009 <para>
1010 The macro <literal>DRD_TRACE_VAR(x)</literal>. Trace all load and store
1011 activity for the address range starting at <literal>&x</literal> and
1012 occupying <literal>sizeof(x)</literal> bytes. When DRD reports a data
1013 race on a specified variable, and it's not immediately clear which
1014 source code statements triggered the conflicting accesses, it can be
1015 very helpful to trace all activity on the offending memory location.
1016 </para>
1017 </listitem>
1018 <listitem>
1019 <para>
1020 The macro <literal>DRD_STOP_TRACING_VAR(x)</literal>. Stop tracing load
1021 and store activity for the address range starting
1022 at <literal>&x</literal> and occupying <literal>sizeof(x)</literal>
1023 bytes.
1024 </para>
1025 </listitem>
1026 <listitem>
1027 <para>
1028 The macro <literal>ANNOTATE_TRACE_MEMORY(&x)</literal>. Trace all
1029 load and store activity that touches at least the single byte at the
1030 address <literal>&x</literal>.
1031 </para>
1032 </listitem>
1033 <listitem>
1034 <para>
1035 The client request <varname>VG_USERREQ__DRD_START_TRACE_ADDR</varname>,
1036 which allows to trace all load and store activity for the specified
1037 address range.
1038 </para>
1039 </listitem>
1040 <listitem>
1041 <para>
1042 The client
1043 request <varname>VG_USERREQ__DRD_STOP_TRACE_ADDR</varname>. Do no longer
1044 trace load and store activity for the specified address range.
1045 </para>
1046 </listitem>
1047 <listitem>
1048 <para>
1049 The macro <literal>ANNOTATE_HAPPENS_BEFORE(addr)</literal> tells DRD to
1050 insert a mark. Insert this macro just after an access to the variable at
1051 the specified address has been performed.
1052 </para>
1053 </listitem>
1054 <listitem>
1055 <para>
1056 The macro <literal>ANNOTATE_HAPPENS_AFTER(addr)</literal> tells DRD that
1057 the next access to the variable at the specified address should be
1058 considered to have happened after the access just before the latest
1059 <literal>ANNOTATE_HAPPENS_BEFORE(addr)</literal> annotation that
1060 references the same variable. The purpose of these two macros is to tell
1061 DRD about the order of inter-thread memory accesses implemented via
1062 atomic memory operations. See
1063 also <literal>drd/tests/annotate_smart_pointer.cpp</literal> for an
1064 example.
1065 </para>
1066 </listitem>
1067 <listitem>
1068 <para>
1069 The macro <literal>ANNOTATE_RWLOCK_CREATE(rwlock)</literal> tells DRD
1070 that the object at address <literal>rwlock</literal> is a
1071 reader-writer synchronization object that is not a
1072 <literal>pthread_rwlock_t</literal> synchronization object. See
1073 also <literal>drd/tests/annotate_rwlock.c</literal> for an example.
1074 </para>
1075 </listitem>
1076 <listitem>
1077 <para>
1078 The macro <literal>ANNOTATE_RWLOCK_DESTROY(rwlock)</literal> tells DRD
1079 that the reader-writer synchronization object at
1080 address <literal>rwlock</literal> has been destroyed.
1081 </para>
1082 </listitem>
1083 <listitem>
1084 <para>
1085 The macro <literal>ANNOTATE_WRITERLOCK_ACQUIRED(rwlock)</literal> tells
1086 DRD that a writer lock has been acquired on the reader-writer
1087 synchronization object at address <literal>rwlock</literal>.
1088 </para>
1089 </listitem>
1090 <listitem>
1091 <para>
1092 The macro <literal>ANNOTATE_READERLOCK_ACQUIRED(rwlock)</literal> tells
1093 DRD that a reader lock has been acquired on the reader-writer
1094 synchronization object at address <literal>rwlock</literal>.
1095 </para>
1096 </listitem>
1097 <listitem>
1098 <para>
1099 The macro <literal>ANNOTATE_RWLOCK_ACQUIRED(rwlock, is_w)</literal>
1100 tells DRD that a writer lock (when <literal>is_w != 0</literal>) or that
1101 a reader lock (when <literal>is_w == 0</literal>) has been acquired on
1102 the reader-writer synchronization object at
1103 address <literal>rwlock</literal>.
1104 </para>
1105 </listitem>
1106 <listitem>
1107 <para>
1108 The macro <literal>ANNOTATE_WRITERLOCK_RELEASED(rwlock)</literal> tells
1109 DRD that a writer lock has been released on the reader-writer
1110 synchronization object at address <literal>rwlock</literal>.
1111 </para>
1112 </listitem>
1113 <listitem>
1114 <para>
1115 The macro <literal>ANNOTATE_READERLOCK_RELEASED(rwlock)</literal> tells
1116 DRD that a reader lock has been released on the reader-writer
1117 synchronization object at address <literal>rwlock</literal>.
1118 </para>
1119 </listitem>
1120 <listitem>
1121 <para>
1122 The macro <literal>ANNOTATE_RWLOCK_RELEASED(rwlock, is_w)</literal>
1123 tells DRD that a writer lock (when <literal>is_w != 0</literal>) or that
1124 a reader lock (when <literal>is_w == 0</literal>) has been released on
1125 the reader-writer synchronization object at
1126 address <literal>rwlock</literal>.
1127 </para>
1128 </listitem>
1129 <listitem>
1130 <para>
1131 The macro <literal>ANNOTATE_BARRIER_INIT(barrier, count,
1132 reinitialization_allowed)</literal> tells DRD that a new barrier object
1133 at the address <literal>barrier</literal> has been initialized,
1134 that <literal>count</literal> threads participate in each barrier and
1135 also whether or not barrier reinitialization without intervening
1136 destruction should be reported as an error. See
1137 also <literal>drd/tests/annotate_barrier.c</literal> for an example.
1138 </para>
1139 </listitem>
1140 <listitem>
1141 <para>
1142 The macro <literal>ANNOTATE_BARRIER_DESTROY(barrier)</literal>
1143 tells DRD that a barrier object is about to be destroyed.
1144 </para>
1145 </listitem>
1146 <listitem>
1147 <para>
1148 The macro <literal>ANNOTATE_BARRIER_WAIT_BEFORE(barrier)</literal>
1149 tells DRD that waiting for a barrier will start.
1150 </para>
1151 </listitem>
1152 <listitem>
1153 <para>
1154 The macro <literal>ANNOTATE_BARRIER_WAIT_AFTER(barrier)</literal>
1155 tells DRD that waiting for a barrier has finished.
1156 </para>
1157 </listitem>
1158 <listitem>
1159 <para>
1160 The macro <literal>ANNOTATE_BENIGN_RACE_SIZED(addr, size,
1161 descr)</literal> tells DRD that any races detected on the specified
1162 address are benign and hence should not be
1163 reported. The <literal>descr</literal> argument is ignored but can be
1164 used to document why data races on <literal>addr</literal> are benign.
1165 </para>
1166 </listitem>
1167 <listitem>
1168 <para>
1169 The macro <literal>ANNOTATE_BENIGN_RACE_STATIC(var, descr)</literal>
1170 tells DRD that any races detected on the specified static variable are
1171 benign and hence should not be reported. The <literal>descr</literal>
1172 argument is ignored but can be used to document why data races
1173 on <literal>var</literal> are benign. Note: this macro can only be
1174 used in C++ programs and not in C programs.
1175 </para>
1176 </listitem>
1177 <listitem>
1178 <para>
1179 The macro <literal>ANNOTATE_IGNORE_READS_BEGIN</literal> tells
1180 DRD to ignore all memory loads performed by the current thread.
1181 </para>
1182 </listitem>
1183 <listitem>
1184 <para>
1185 The macro <literal>ANNOTATE_IGNORE_READS_END</literal> tells
1186 DRD to stop ignoring the memory loads performed by the current thread.
1187 </para>
1188 </listitem>
1189 <listitem>
1190 <para>
1191 The macro <literal>ANNOTATE_IGNORE_WRITES_BEGIN</literal> tells
1192 DRD to ignore all memory stores performed by the current thread.
1193 </para>
1194 </listitem>
1195 <listitem>
1196 <para>
1197 The macro <literal>ANNOTATE_IGNORE_WRITES_END</literal> tells
1198 DRD to stop ignoring the memory stores performed by the current thread.
1199 </para>
1200 </listitem>
1201 <listitem>
1202 <para>
1203 The macro <literal>ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN</literal> tells
1204 DRD to ignore all memory accesses performed by the current thread.
1205 </para>
1206 </listitem>
1207 <listitem>
1208 <para>
1209 The macro <literal>ANNOTATE_IGNORE_READS_AND_WRITES_END</literal> tells
1210 DRD to stop ignoring the memory accesses performed by the current thread.
1211 </para>
1212 </listitem>
1213 <listitem>
1214 <para>
1215 The macro <literal>ANNOTATE_NEW_MEMORY(addr, size)</literal> tells
1216 DRD that the specified memory range has been allocated by a custom
1217 memory allocator in the client program and that the client program
1218 will start using this memory range.
1219 </para>
1220 </listitem>
1221 <listitem>
1222 <para>
1223 The macro <literal>ANNOTATE_THREAD_NAME(name)</literal> tells DRD to
1224 associate the specified name with the current thread and to include this
1225 name in the error messages printed by DRD.
1226 </para>
1227 </listitem>
1228 <listitem>
1229 <para>
1230 The macros <literal>VALGRIND_MALLOCLIKE_BLOCK</literal> and
1231 <literal>VALGRIND_FREELIKE_BLOCK</literal> from the Valgrind core are
1232 implemented; they are described in
1233 <xref linkend="manual-core-adv.clientreq"/>.
1234 </para>
1235 </listitem>
1236 </itemizedlist>
1237 </para>
1239 <para>
1240 Note: if you compiled Valgrind yourself, the header file
1241 <literal><valgrind/drd.h></literal> will have been installed in
1242 the directory <literal>/usr/include</literal> by the command
1243 <literal>make install</literal>. If you obtained Valgrind by
1244 installing it as a package however, you will probably have to install
1245 another package with a name like <literal>valgrind-devel</literal>
1246 before Valgrind's header files are available.
1247 </para>
1249 </sect2>
1252 <sect2 id="drd-manual.C++11" xreflabel="C++11">
1253 <title>Debugging C++11 Programs</title>
1255 <para>If you want to use the C++11 class std::thread you will need to do the
1256 following to annotate the std::shared_ptr<> objects used in the
1257 implementation of that class:
1258 <itemizedlist>
1259 <listitem>
1260 <para>Add the following code at the start of a common header or at the
1261 start of each source file, before any C++ header files are included:</para>
1262 <programlisting>
1263 #include <valgrind/drd.h>
1264 #define _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(addr) ANNOTATE_HAPPENS_BEFORE(addr)
1265 #define _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(addr) ANNOTATE_HAPPENS_AFTER(addr)
1266 </programlisting>
1267 </listitem>
1268 <listitem>
1269 <para>Download the gcc source code and from source file
1270 libstdc++-v3/src/c++11/thread.cc copy the implementation of the
1271 <computeroutput>execute_native_thread_routine()</computeroutput>
1272 and <computeroutput>std::thread::_M_start_thread()</computeroutput>
1273 functions into a source file that is linked with your application. Make
1274 sure that also in this source file the
1275 _GLIBCXX_SYNCHRONIZATION_HAPPENS_*() macros are defined properly.</para>
1276 </listitem>
1277 </itemizedlist>
1278 </para>
1279 <para>For more information, see also <emphasis>The
1280 GNU C++ Library Manual, Debugging Support</emphasis>
1281 (<ulink url="http://gcc.gnu.org/onlinedocs/libstdc++/manual/debug.html">http://gcc.gnu.org/onlinedocs/libstdc++/manual/debug.html</ulink>).</para>
1283 </sect2>
1286 <sect2 id="drd-manual.gnome" xreflabel="GNOME">
1287 <title>Debugging GNOME Programs</title>
1289 <para>
1290 GNOME applications use the threading primitives provided by the
1291 <computeroutput>glib</computeroutput> and
1292 <computeroutput>gthread</computeroutput> libraries. These libraries
1293 are built on top of POSIX threads, and hence are directly supported by
1294 DRD. Please keep in mind that you have to call
1295 <function>g_thread_init</function> before creating any threads, or
1296 DRD will report several data races on glib functions. See also the
1297 <ulink
1298 url="http://library.gnome.org/devel/glib/stable/glib-Threads.html">GLib
1299 Reference Manual</ulink> for more information about
1300 <function>g_thread_init</function>.
1301 </para>
1303 <para>
1304 One of the many facilities provided by the <literal>glib</literal>
1305 library is a block allocator, called <literal>g_slice</literal>. You
1306 have to disable this block allocator when using DRD by adding the
1307 following to the shell environment variables:
1308 <literal>G_SLICE=always-malloc</literal>. See also the <ulink
1309 url="http://library.gnome.org/devel/glib/stable/glib-Memory-Slices.html">GLib
1310 Reference Manual</ulink> for more information.
1311 </para>
1313 </sect2>
1316 <sect2 id="drd-manual.boost.thread" xreflabel="Boost.Thread">
1317 <title>Debugging Boost.Thread Programs</title>
1319 <para>
1320 The Boost.Thread library is the threading library included with the
1321 cross-platform Boost Libraries. This threading library is an early
1322 implementation of the upcoming C++0x threading library.
1323 </para>
1325 <para>
1326 Applications that use the Boost.Thread library should run fine under DRD.
1327 </para>
1329 <para>
1330 More information about Boost.Thread can be found here:
1331 <itemizedlist>
1332 <listitem>
1333 <para>
1334 Anthony Williams, <ulink
1335 url="http://www.boost.org/doc/libs/1_37_0/doc/html/thread.html">Boost.Thread</ulink>
1336 Library Documentation, Boost website, 2007.
1337 </para>
1338 </listitem>
1339 <listitem>
1340 <para>
1341 Anthony Williams, <ulink
1342 url="http://www.ddj.com/cpp/211600441">What's New in Boost
1343 Threads?</ulink>, Recent changes to the Boost Thread library,
1344 Dr. Dobbs Magazine, October 2008.
1345 </para>
1346 </listitem>
1347 </itemizedlist>
1348 </para>
1350 </sect2>
1353 <sect2 id="drd-manual.openmp" xreflabel="OpenMP">
1354 <title>Debugging OpenMP Programs</title>
1356 <para>
1357 OpenMP stands for <emphasis>Open Multi-Processing</emphasis>. The OpenMP
1358 standard consists of a set of compiler directives for C, C++ and Fortran
1359 programs that allows a compiler to transform a sequential program into a
1360 parallel program. OpenMP is well suited for HPC applications and allows to
1361 work at a higher level compared to direct use of the POSIX threads API. While
1362 OpenMP ensures that the POSIX API is used correctly, OpenMP programs can still
1363 contain data races. So it definitely makes sense to verify OpenMP programs
1364 with a thread checking tool.
1365 </para>
1367 <para>
1368 DRD supports OpenMP shared-memory programs generated by GCC. GCC
1369 supports OpenMP since version 4.2.0. GCC's runtime support
1370 for OpenMP programs is provided by a library called
1371 <literal>libgomp</literal>. The synchronization primitives implemented
1372 in this library use Linux' futex system call directly, unless the
1373 library has been configured with the
1374 <literal>--disable-linux-futex</literal> option. DRD only supports
1375 libgomp libraries that have been configured with this option and in
1376 which symbol information is present. For most Linux distributions this
1377 means that you will have to recompile GCC. See also the script
1378 <literal>drd/scripts/download-and-build-gcc</literal> in the
1379 Valgrind source tree for an example of how to compile GCC. You will
1380 also have to make sure that the newly compiled
1381 <literal>libgomp.so</literal> library is loaded when OpenMP programs
1382 are started. This is possible by adding a line similar to the
1383 following to your shell startup script:
1384 </para>
1385 <programlisting><![CDATA[
1386 export LD_LIBRARY_PATH=~/gcc-4.4.0/lib64:~/gcc-4.4.0/lib:
1387 ]]></programlisting>
1389 <para>
1390 As an example, the test OpenMP test program
1391 <literal>drd/tests/omp_matinv</literal> triggers a data race
1392 when the option -r has been specified on the command line. The data
1393 race is triggered by the following code:
1394 </para>
1395 <programlisting><![CDATA[
1396 #pragma omp parallel for private(j)
1397 for (j = 0; j < rows; j++)
1398 {
1399 if (i != j)
1400 {
1401 const elem_t factor = a[j * cols + i];
1402 for (k = 0; k < cols; k++)
1403 {
1404 a[j * cols + k] -= a[i * cols + k] * factor;
1405 }
1406 }
1407 }
1408 ]]></programlisting>
1410 <para>
1411 The above code is racy because the variable <literal>k</literal> has
1412 not been declared private. DRD will print the following error message
1413 for the above code:
1414 </para>
1415 <programlisting><![CDATA[
1416 $ valgrind --tool=drd --check-stack-var=yes --read-var-info=yes drd/tests/omp_matinv 3 -t 2 -r
1417 ...
1418 Conflicting store by thread 1/1 at 0x7fefffbc4 size 4
1419 at 0x4014A0: gj.omp_fn.0 (omp_matinv.c:203)
1420 by 0x401211: gj (omp_matinv.c:159)
1421 by 0x40166A: invert_matrix (omp_matinv.c:238)
1422 by 0x4019B4: main (omp_matinv.c:316)
1423 Location 0x7fefffbc4 is 0 bytes inside local var "k"
1424 declared at omp_matinv.c:160, in frame #0 of thread 1
1425 ...
1426 ]]></programlisting>
1427 <para>
1428 In the above output the function name <function>gj.omp_fn.0</function>
1429 has been generated by GCC from the function name
1430 <function>gj</function>. The allocation context information shows that the
1431 data race has been caused by modifying the variable <literal>k</literal>.
1432 </para>
1434 <para>
1435 Note: for GCC versions before 4.4.0, no allocation context information is
1436 shown. With these GCC versions the most usable information in the above output
1437 is the source file name and the line number where the data race has been
1438 detected (<literal>omp_matinv.c:203</literal>).
1439 </para>
1441 <para>
1442 For more information about OpenMP, see also
1443 <ulink url="http://openmp.org/">openmp.org</ulink>.
1444 </para>
1446 </sect2>
1449 <sect2 id="drd-manual.cust-mem-alloc" xreflabel="Custom Memory Allocators">
1450 <title>DRD and Custom Memory Allocators</title>
1452 <para>
1453 DRD tracks all memory allocation events that happen via the
1454 standard memory allocation and deallocation functions
1455 (<function>malloc</function>, <function>free</function>,
1456 <function>new</function> and <function>delete</function>), via entry
1457 and exit of stack frames or that have been annotated with Valgrind's
1458 memory pool client requests. DRD uses memory allocation and deallocation
1459 information for two purposes:
1460 <itemizedlist>
1461 <listitem>
1462 <para>
1463 To know where the scope ends of POSIX objects that have not been
1464 destroyed explicitly. It is e.g. not required by the POSIX
1465 threads standard to call
1466 <function>pthread_mutex_destroy</function> before freeing the
1467 memory in which a mutex object resides.
1468 </para>
1469 </listitem>
1470 <listitem>
1471 <para>
1472 To know where the scope of variables ends. If e.g. heap memory
1473 has been used by one thread, that thread frees that memory, and
1474 another thread allocates and starts using that memory, no data
1475 races must be reported for that memory.
1476 </para>
1477 </listitem>
1478 </itemizedlist>
1479 </para>
1481 <para>
1482 It is essential for correct operation of DRD that the tool knows about
1483 memory allocation and deallocation events. When analyzing a client program
1484 with DRD that uses a custom memory allocator, either instrument the custom
1485 memory allocator with the <literal>VALGRIND_MALLOCLIKE_BLOCK</literal>
1486 and <literal>VALGRIND_FREELIKE_BLOCK</literal> macros or disable the
1487 custom memory allocator.
1488 </para>
1490 <para>
1491 As an example, the GNU libstdc++ library can be configured
1492 to use standard memory allocation functions instead of memory pools by
1493 setting the environment variable
1494 <literal>GLIBCXX_FORCE_NEW</literal>. For more information, see also
1495 the <ulink
1496 url="http://gcc.gnu.org/onlinedocs/libstdc++/manual/bk01pt04ch11.html">libstdc++
1497 manual</ulink>.
1498 </para>
1500 </sect2>
1503 <sect2 id="drd-manual.drd-versus-memcheck" xreflabel="DRD Versus Memcheck">
1504 <title>DRD Versus Memcheck</title>
1506 <para>
1507 It is essential for correct operation of DRD that there are no memory
1508 errors such as dangling pointers in the client program. Which means that
1509 it is a good idea to make sure that your program is Memcheck-clean
1510 before you analyze it with DRD. It is possible however that some of
1511 the Memcheck reports are caused by data races. In this case it makes
1512 sense to run DRD before Memcheck.
1513 </para>
1515 <para>
1516 So which tool should be run first? In case both DRD and Memcheck
1517 complain about a program, a possible approach is to run both tools
1518 alternatingly and to fix as many errors as possible after each run of
1519 each tool until none of the two tools prints any more error messages.
1520 </para>
1522 </sect2>
1525 <sect2 id="drd-manual.resource-requirements" xreflabel="Resource Requirements">
1526 <title>Resource Requirements</title>
1528 <para>
1529 The requirements of DRD with regard to heap and stack memory and the
1530 effect on the execution time of client programs are as follows:
1531 <itemizedlist>
1532 <listitem>
1533 <para>
1534 When running a program under DRD with default DRD options,
1535 between 1.1 and 3.6 times more memory will be needed compared to
1536 a native run of the client program. More memory will be needed
1537 if loading debug information has been enabled
1538 (<literal>--read-var-info=yes</literal>).
1539 </para>
1540 </listitem>
1541 <listitem>
1542 <para>
1543 DRD allocates some of its temporary data structures on the stack
1544 of the client program threads. This amount of data is limited to
1545 1 - 2 KB. Make sure that thread stacks are sufficiently large.
1546 </para>
1547 </listitem>
1548 <listitem>
1549 <para>
1550 Most applications will run between 20 and 50 times slower under
1551 DRD than a native single-threaded run. The slowdown will be most
1552 noticeable for applications which perform frequent mutex lock /
1553 unlock operations.
1554 </para>
1555 </listitem>
1556 </itemizedlist>
1557 </para>
1559 </sect2>
1562 <sect2 id="drd-manual.effective-use" xreflabel="Effective Use">
1563 <title>Hints and Tips for Effective Use of DRD</title>
1565 <para>
1566 The following information may be helpful when using DRD:
1567 <itemizedlist>
1568 <listitem>
1569 <para>
1570 Make sure that debug information is present in the executable
1571 being analyzed, such that DRD can print function name and line
1572 number information in stack traces. Most compilers can be told
1573 to include debug information via compiler option
1574 <option>-g</option>.
1575 </para>
1576 </listitem>
1577 <listitem>
1578 <para>
1579 Compile with option <option>-O1</option> instead of
1580 <option>-O0</option>. This will reduce the amount of generated
1581 code, may reduce the amount of debug info and will speed up
1582 DRD's processing of the client program. For more information,
1583 see also <xref linkend="manual-core.started"/>.
1584 </para>
1585 </listitem>
1586 <listitem>
1587 <para>
1588 If DRD reports any errors on libraries that are part of your
1589 Linux distribution like e.g. <literal>libc.so</literal> or
1590 <literal>libstdc++.so</literal>, installing the debug packages
1591 for these libraries will make the output of DRD a lot more
1592 detailed.
1593 </para>
1594 </listitem>
1595 <listitem>
1596 <para>
1597 When using C++, do not send output from more than one thread to
1598 <literal>std::cout</literal>. Doing so would not only
1599 generate multiple data race reports, it could also result in
1600 output from several threads getting mixed up. Either use
1601 <function>printf</function> or do the following:
1602 <orderedlist>
1603 <listitem>
1604 <para>Derive a class from <literal>std::ostreambuf</literal>
1605 and let that class send output line by line to
1606 <literal>stdout</literal>. This will avoid that individual
1607 lines of text produced by different threads get mixed
1608 up.</para>
1609 </listitem>
1610 <listitem>
1611 <para>Create one instance of <literal>std::ostream</literal>
1612 for each thread. This makes stream formatting settings
1613 thread-local. Pass a per-thread instance of the class
1614 derived from <literal>std::ostreambuf</literal> to the
1615 constructor of each instance. </para>
1616 </listitem>
1617 <listitem>
1618 <para>Let each thread send its output to its own instance of
1619 <literal>std::ostream</literal> instead of
1620 <literal>std::cout</literal>.</para>
1621 </listitem>
1622 </orderedlist>
1623 </para>
1624 </listitem>
1625 </itemizedlist>
1626 </para>
1628 </sect2>
1631 </sect1>
1634 <sect1 id="drd-manual.Pthreads" xreflabel="Pthreads">
1635 <title>Using the POSIX Threads API Effectively</title>
1637 <sect2 id="drd-manual.mutex-types" xreflabel="mutex-types">
1638 <title>Mutex types</title>
1640 <para>
1641 The Single UNIX Specification version two defines the following four
1642 mutex types (see also the documentation of <ulink
1643 url="http://www.opengroup.org/onlinepubs/007908799/xsh/pthread_mutexattr_settype.html"><function>pthread_mutexattr_settype</function></ulink>):
1644 <itemizedlist>
1645 <listitem>
1646 <para>
1647 <emphasis>normal</emphasis>, which means that no error checking
1648 is performed, and that the mutex is non-recursive.
1649 </para>
1650 </listitem>
1651 <listitem>
1652 <para>
1653 <emphasis>error checking</emphasis>, which means that the mutex
1654 is non-recursive and that error checking is performed.
1655 </para>
1656 </listitem>
1657 <listitem>
1658 <para>
1659 <emphasis>recursive</emphasis>, which means that a mutex may be
1660 locked recursively.
1661 </para>
1662 </listitem>
1663 <listitem>
1664 <para>
1665 <emphasis>default</emphasis>, which means that error checking
1666 behavior is undefined, and that the behavior for recursive
1667 locking is also undefined. Or: portable code must neither
1668 trigger error conditions through the Pthreads API nor attempt to
1669 lock a mutex of default type recursively.
1670 </para>
1671 </listitem>
1672 </itemizedlist>
1673 </para>
1675 <para>
1676 In complex applications it is not always clear from beforehand which
1677 mutex will be locked recursively and which mutex will not be locked
1678 recursively. Attempts lock a non-recursive mutex recursively will
1679 result in race conditions that are very hard to find without a thread
1680 checking tool. So either use the error checking mutex type and
1681 consistently check the return value of Pthread API mutex calls, or use
1682 the recursive mutex type.
1683 </para>
1685 </sect2>
1687 <sect2 id="drd-manual.condvar" xreflabel="condition-variables">
1688 <title>Condition variables</title>
1690 <para>
1691 A condition variable allows one thread to wake up one or more other
1692 threads. Condition variables are often used to notify one or more
1693 threads about state changes of shared data. Unfortunately it is very
1694 easy to introduce race conditions by using condition variables as the
1695 only means of state information propagation. A better approach is to
1696 let threads poll for changes of a state variable that is protected by
1697 a mutex, and to use condition variables only as a thread wakeup
1698 mechanism. See also the source file
1699 <computeroutput>drd/tests/monitor_example.cpp</computeroutput> for an
1700 example of how to implement this concept in C++. The monitor concept
1701 used in this example is a well known and very useful concept -- see
1702 also Wikipedia for more information about the <ulink
1703 url="http://en.wikipedia.org/wiki/Monitor_(synchronization)">monitor</ulink>
1704 concept.
1705 </para>
1707 </sect2>
1709 <sect2 id="drd-manual.pctw" xreflabel="pthread_cond_timedwait">
1710 <title>pthread_cond_timedwait and timeouts</title>
1712 <para>
1713 Historically the function
1714 <function>pthread_cond_timedwait</function> only allowed the
1715 specification of an absolute timeout, that is a timeout independent of
1716 the time when this function was called. However, almost every call to
1717 this function expresses a relative timeout. This typically happens by
1718 passing the sum of
1719 <computeroutput>clock_gettime(CLOCK_REALTIME)</computeroutput> and a
1720 relative timeout as the third argument. This approach is incorrect
1721 since forward or backward clock adjustments by e.g. ntpd will affect
1722 the timeout. A more reliable approach is as follows:
1723 <itemizedlist>
1724 <listitem>
1725 <para>
1726 When initializing a condition variable through
1727 <function>pthread_cond_init</function>, specify that the timeout of
1728 <function>pthread_cond_timedwait</function> will use the clock
1729 <literal>CLOCK_MONOTONIC</literal> instead of
1730 <literal>CLOCK_REALTIME</literal>. You can do this via
1731 <computeroutput>pthread_condattr_setclock(...,
1732 CLOCK_MONOTONIC)</computeroutput>.
1733 </para>
1734 </listitem>
1735 <listitem>
1736 <para>
1737 When calling <function>pthread_cond_timedwait</function>, pass
1738 the sum of
1739 <computeroutput>clock_gettime(CLOCK_MONOTONIC)</computeroutput>
1740 and a relative timeout as the third argument.
1741 </para>
1742 </listitem>
1743 </itemizedlist>
1744 See also
1745 <computeroutput>drd/tests/monitor_example.cpp</computeroutput> for an
1746 example.
1747 </para>
1749 </sect2>
1751 </sect1>
1754 <sect1 id="drd-manual.limitations" xreflabel="Limitations">
1755 <title>Limitations</title>
1757 <para>DRD currently has the following limitations:</para>
1759 <itemizedlist>
1760 <listitem>
1761 <para>
1762 DRD, just like Memcheck, will refuse to start on Linux
1763 distributions where all symbol information has been removed from
1764 <filename>ld.so</filename>. This is e.g. the case for the PPC editions
1765 of openSUSE and Gentoo. You will have to install the glibc debuginfo
1766 package on these platforms before you can use DRD. See also openSUSE
1767 bug <ulink url="http://bugzilla.novell.com/show_bug.cgi?id=396197">
1768 396197</ulink> and Gentoo bug <ulink
1769 url="http://bugs.gentoo.org/214065">214065</ulink>.
1770 </para>
1771 </listitem>
1772 <listitem>
1773 <para>
1774 With gcc 4.4.3 and before, DRD may report data races on the C++
1775 class <literal>std::string</literal> in a multithreaded program. This is
1776 a know <literal>libstdc++</literal> issue -- see also GCC bug
1777 <ulink url="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40518">40518</ulink>
1778 for more information.
1779 </para>
1780 </listitem>
1781 <listitem>
1782 <para>
1783 If you compile the DRD source code yourself, you need GCC 3.0 or
1784 later. GCC 2.95 is not supported.
1785 </para>
1786 </listitem>
1787 <listitem>
1788 <para>
1789 Of the two POSIX threads implementations for Linux, only the
1790 NPTL (Native POSIX Thread Library) is supported. The older
1791 LinuxThreads library is not supported.
1792 </para>
1793 </listitem>
1794 </itemizedlist>
1796 </sect1>
1799 <sect1 id="drd-manual.feedback" xreflabel="Feedback">
1800 <title>Feedback</title>
1802 <para>
1803 If you have any comments, suggestions, feedback or bug reports about
1804 DRD, feel free to either post a message on the Valgrind users mailing
1805 list or to file a bug report. See also <ulink
1806 url="&vg-url;">&vg-url;</ulink> for more information.
1807 </para>
1809 </sect1>
1812 </chapter>