1 Using RCU (Read-Copy-Update) for synchronization
2 ================================================
4 Read-copy update (RCU) is a synchronization mechanism that is used to
5 protect read-mostly data structures. RCU is very efficient and scalable
6 on the read side (it is wait-free), and thus can make the read paths
9 RCU supports concurrency between a single writer and multiple readers,
10 thus it is not used alone. Typically, the write-side will use a lock to
11 serialize multiple updates, but other approaches are possible (e.g.,
12 restricting updates to a single task). In QEMU, when a lock is used,
13 this will often be the "iothread mutex", also known as the "big QEMU
14 lock" (BQL). Also, restricting updates to a single task is done in
15 QEMU using the "bottom half" API.
17 RCU is fundamentally a "wait-to-finish" mechanism. The read side marks
18 sections of code with "critical sections", and the update side will wait
19 for the execution of all *currently running* critical sections before
20 proceeding, or before asynchronously executing a callback.
22 The key point here is that only the currently running critical sections
23 are waited for; critical sections that are started **after** the beginning
24 of the wait do not extend the wait, despite running concurrently with
25 the updater. This is the reason why RCU is more scalable than,
26 for example, reader-writer locks. It is so much more scalable that
27 the system will have a single instance of the RCU mechanism; a single
28 mechanism can be used for an arbitrary number of "things", without
29 having to worry about things such as contention or deadlocks.
31 How is this possible? The basic idea is to split updates in two phases,
32 "removal" and "reclamation". During removal, we ensure that subsequent
33 readers will not be able to get a reference to the old data. After
34 removal has completed, a critical section will not be able to access
35 the old data. Therefore, critical sections that begin after removal
36 do not matter; as soon as all previous critical sections have finished,
37 there cannot be any readers who hold references to the data structure,
38 and these can now be safely reclaimed (e.g., freed or unref'ed).
42 thread 1 thread 2 thread 3
43 ------------------- ------------------------ -------------------
45 | finish removal phase
47 | | enter RCU crit.sec.
50 begin reclamation phase |
54 Note how thread 3 is still executing its critical section when thread 2
55 starts reclaiming data. This is possible, because the old version of the
56 data structure was not accessible at the time thread 3 began executing
57 that critical section.
63 The core RCU API is small:
65 ``void rcu_read_lock(void);``
66 Used by a reader to inform the reclaimer that the reader is
67 entering an RCU read-side critical section.
69 ``void rcu_read_unlock(void);``
70 Used by a reader to inform the reclaimer that the reader is
71 exiting an RCU read-side critical section. Note that RCU
72 read-side critical sections may be nested and/or overlapping.
74 ``void synchronize_rcu(void);``
75 Blocks until all pre-existing RCU read-side critical sections
76 on all threads have completed. This marks the end of the removal
77 phase and the beginning of reclamation phase.
79 Note that it would be valid for another update to come while
80 ``synchronize_rcu`` is running. Because of this, it is better that
81 the updater releases any locks it may hold before calling
82 ``synchronize_rcu``. If this is not possible (for example, because
83 the updater is protected by the BQL), you can use ``call_rcu``.
85 ``void call_rcu1(struct rcu_head * head, void (*func)(struct rcu_head *head));``
86 This function invokes ``func(head)`` after all pre-existing RCU
87 read-side critical sections on all threads have completed. This
88 marks the end of the removal phase, with func taking care
89 asynchronously of the reclamation phase.
91 The ``foo`` struct needs to have an ``rcu_head`` structure added,
101 so that the reclaimer function can fetch the ``struct foo`` address
104 call_rcu1(&foo.rcu, foo_reclaim);
106 void foo_reclaim(struct rcu_head *rp)
108 struct foo *fp = container_of(rp, struct foo, rcu);
112 ``call_rcu1`` is typically used via either the ``call_rcu`` or
113 ``g_free_rcu`` macros, which handle the common case where the
114 ``rcu_head`` member is the first of the struct.
116 ``void call_rcu(T *p, void (*func)(T *p), field-name);``
117 If the ``struct rcu_head`` is the first field in the struct, you can
118 use this macro instead of ``call_rcu1``.
120 ``void g_free_rcu(T *p, field-name);``
121 This is a special-case version of ``call_rcu`` where the callback
122 function is ``g_free``.
123 In the example given in ``call_rcu1``, one could have written simply::
125 g_free_rcu(&foo, rcu);
127 ``typeof(*p) qatomic_rcu_read(p);``
128 ``qatomic_rcu_read()`` is similar to ``qatomic_load_acquire()``, but
129 it makes some assumptions on the code that calls it. This allows a
130 more optimized implementation.
132 ``qatomic_rcu_read`` assumes that whenever a single RCU critical
133 section reads multiple shared data, these reads are either
134 data-dependent or need no ordering. This is almost always the
135 case when using RCU, because read-side critical sections typically
136 navigate one or more pointers (the pointers that are changed on
137 every update) until reaching a data structure of interest,
138 and then read from there.
140 RCU read-side critical sections must use ``qatomic_rcu_read()`` to
141 read data, unless concurrent writes are prevented by another
142 synchronization mechanism.
144 Furthermore, RCU read-side critical sections should traverse the
145 data structure in a single direction, opposite to the direction
146 in which the updater initializes it.
148 ``void qatomic_rcu_set(p, typeof(*p) v);``
149 ``qatomic_rcu_set()`` is similar to ``qatomic_store_release()``,
150 though it also makes assumptions on the code that calls it in
151 order to allow a more optimized implementation.
153 In particular, ``qatomic_rcu_set()`` suffices for synchronization
154 with readers, if the updater never mutates a field within a
155 data item that is already accessible to readers. This is the
156 case when initializing a new copy of the RCU-protected data
157 structure; just ensure that initialization of ``*p`` is carried out
158 before ``qatomic_rcu_set()`` makes the data item visible to readers.
159 If this rule is observed, writes will happen in the opposite
160 order as reads in the RCU read-side critical sections (or if
161 there is just one update), and there will be no need for other
162 synchronization mechanism to coordinate the accesses.
164 The following APIs must be used before RCU is used in a thread:
166 ``void rcu_register_thread(void);``
167 Mark a thread as taking part in the RCU mechanism. Such a thread
168 will have to report quiescent points regularly, either manually
169 or through the ``QemuCond``/``QemuSemaphore``/``QemuEvent`` APIs.
171 ``void rcu_unregister_thread(void);``
172 Mark a thread as not taking part anymore in the RCU mechanism.
173 It is not a problem if such a thread reports quiescent points,
174 either manually or by using the
175 ``QemuCond``/``QemuSemaphore``/``QemuEvent`` APIs.
177 Note that these APIs are relatively heavyweight, and should **not** be
183 Two macros are provided that automatically release the read lock at the
186 ``RCU_READ_LOCK_GUARD()``
187 Takes the lock and will release it at the end of the block it's
190 ``WITH_RCU_READ_LOCK_GUARD() { code }``
191 Is used at the head of a block to protect the code within the block.
193 Note that a ``goto`` out of the guarded block will also drop the lock.
195 Differences with Linux
196 ----------------------
198 - Waiting on a mutex is possible, though discouraged, within an RCU critical
199 section. This is because spinlocks are rarely (if ever) used in userspace
200 programming; not allowing this would prevent upgrading an RCU read-side
201 critical section to become an updater.
203 - ``qatomic_rcu_read`` and ``qatomic_rcu_set`` replace ``rcu_dereference`` and
204 ``rcu_assign_pointer``. They take a **pointer** to the variable being accessed.
206 - ``call_rcu`` is a macro that has an extra argument (the name of the first
207 field in the struct, which must be a struct ``rcu_head``), and expects the
208 type of the callback's argument to be the type of the first argument.
209 ``call_rcu1`` is the same as Linux's ``call_rcu``.
215 Many patterns using read-writer locks translate directly to RCU, with
216 the advantages of higher scalability and deadlock immunity.
218 In general, RCU can be used whenever it is possible to create a new
219 "version" of a data structure every time the updater runs. This may
220 sound like a very strict restriction, however:
222 - the updater does not mean "everything that writes to a data structure",
223 but rather "everything that involves a reclamation step". See the
226 - in some cases, creating a new version of a data structure may actually
227 be very cheap. For example, modifying the "next" pointer of a singly
228 linked list is effectively creating a new version of the list.
230 Here are some frequently-used RCU idioms that are worth noting.
236 TBD (not yet used in QEMU)
239 RCU reference counting
240 ^^^^^^^^^^^^^^^^^^^^^^
242 Because grace periods are not allowed to complete while there is an RCU
243 read-side critical section in progress, the RCU read-side primitives
244 may be used as a restricted reference-counting mechanism. For example,
245 consider the following code fragment::
248 p = qatomic_rcu_read(&foo);
249 /* do something with p. */
252 The RCU read-side critical section ensures that the value of ``p`` remains
253 valid until after the ``rcu_read_unlock()``. In some sense, it is acquiring
254 a reference to ``p`` that is later released when the critical section ends.
255 The write side looks simply like this (with appropriate locking)::
257 qemu_mutex_lock(&foo_mutex);
259 qatomic_rcu_set(&foo, new);
260 qemu_mutex_unlock(&foo_mutex);
264 If the processing cannot be done purely within the critical section, it
265 is possible to combine this idiom with a "real" reference count::
268 p = qatomic_rcu_read(&foo);
271 /* do something with p. */
274 The write side can be like this::
276 qemu_mutex_lock(&foo_mutex);
278 qatomic_rcu_set(&foo, new);
279 qemu_mutex_unlock(&foo_mutex);
283 or with ``call_rcu``::
285 qemu_mutex_lock(&foo_mutex);
287 qatomic_rcu_set(&foo, new);
288 qemu_mutex_unlock(&foo_mutex);
289 call_rcu(foo_unref, old, rcu);
291 In both cases, the write side only performs removal. Reclamation
292 happens when the last reference to a ``foo`` object is dropped.
293 Using ``synchronize_rcu()`` is undesirably expensive, because the
294 last reference may be dropped on the read side. Hence you can
295 use ``call_rcu()`` instead::
297 foo_unref(struct foo *p) {
298 if (qatomic_fetch_dec(&p->refcount) == 1) {
299 call_rcu(foo_destroy, p, rcu);
304 Note that the same idioms would be possible with reader/writer
307 read_lock(&foo_rwlock); write_mutex_lock(&foo_rwlock);
309 /* do something with p. */ foo = new;
310 read_unlock(&foo_rwlock); free(p);
311 write_mutex_unlock(&foo_rwlock);
314 ------------------------------------------------------------------
316 read_lock(&foo_rwlock); write_mutex_lock(&foo_rwlock);
318 foo_ref(p); foo = new;
319 read_unlock(&foo_rwlock); foo_unref(old);
320 /* do something with p. */ write_mutex_unlock(&foo_rwlock);
321 read_lock(&foo_rwlock);
323 read_unlock(&foo_rwlock);
325 ``foo_unref`` could use a mechanism such as bottom halves to move deallocation
326 out of the write-side critical section.
332 Resizable arrays can be used with RCU. The expensive RCU synchronization
333 (or ``call_rcu``) only needs to take place when the array is resized.
334 The two items to take care of are:
336 - ensuring that the old version of the array is available between removal
339 - avoiding mismatches in the read side between the array data and the
342 The first problem is avoided simply by not using ``realloc``. Instead,
343 each resize will allocate a new array and copy the old data into it.
344 The second problem would arise if the size and the data pointers were
345 two members of a larger struct::
355 Instead, we store the size of the array with the array itself::
362 struct arr *global_array;
366 struct arr *array = qatomic_rcu_read(&global_array);
367 x = i < array->size ? array->data[i] : -1;
371 write side (running under a lock):
372 if (global_array->size == global_array->alloc) {
373 /* Creating a new version. */
374 new_array = g_malloc(sizeof(struct arr) +
375 global_array->alloc * 2 * sizeof(T));
376 new_array->size = global_array->size;
377 new_array->alloc = global_array->alloc * 2;
378 memcpy(new_array->data, global_array->data,
379 global_array->alloc * sizeof(T));
382 old_array = global_array;
383 qatomic_rcu_set(&global_array, new_array);
386 /* Reclamation phase. */
394 * The `Linux kernel RCU documentation <https://docs.kernel.org/RCU/>`__