1 completions - wait for completion handling
2 ==========================================
4 This document was originally written based on 3.18.0 (linux-next)
9 If you have one or more threads of execution that must wait for some process
10 to have reached a point or a specific state, completions can provide a race
11 free solution to this problem. Semantically they are somewhat like a
12 pthread_barriers and have similar use-cases.
14 Completions are a code synchronization mechanism that is preferable to any
15 misuse of locks. Any time you think of using yield() or some quirky
16 msleep(1); loop to allow something else to proceed, you probably want to
17 look into using one of the wait_for_completion*() calls instead. The
18 advantage of using completions is clear intent of the code but also more
19 efficient code as both threads can continue until the result is actually
22 Completions are built on top of the generic event infrastructure in Linux,
23 with the event reduced to a simple flag appropriately called "done" in
24 struct completion, that tells the waiting threads of execution if they
27 As completions are scheduling related the code is found in
28 kernel/sched/completion.c - for details on completion design and
29 implementation see completions-design.txt
35 There are three parts to the using completions, the initialization of the
36 struct completion, the waiting part through a call to one of the variants of
37 wait_for_completion() and the signaling side through a call to complete(),
38 or complete_all(). Further there are some helper functions for checking the
41 To use completions one needs to include <linux/completion.h> and
42 create a variable of type struct completion. The structure used for
43 handling of completions is:
47 wait_queue_head_t wait;
50 providing the wait queue to place tasks on for waiting and the flag for
51 indicating the state of affairs.
53 Completions should be named to convey the intent of the waiter. A good
56 wait_for_completion(&early_console_added);
58 complete(&early_console_added);
60 Good naming (as always) helps code readability.
63 Initializing completions:
64 -------------------------
66 Initialization of dynamically allocated completions, often embedded in
67 other structures, is done with:
69 void init_completion(&done);
71 Initialization is accomplished by initializing the wait queue and setting
72 the default state to "not available", that is, "done" is set to 0.
74 The re-initialization function, reinit_completion(), simply resets the
75 done element to "not available", thus again to 0, without touching the
76 wait queue. Calling init_completion() on the same completions object is
77 most likely a bug as it re-initializes the queue to an empty queue and
78 enqueued tasks could get "lost" - use reinit_completion() in that case.
80 For static declaration and initialization, macros are available. These are:
82 static DECLARE_COMPLETION(setup_done)
84 used for static declarations in file scope. Within functions the static
85 initialization should always use:
87 DECLARE_COMPLETION_ONSTACK(setup_done)
89 suitable for automatic/local variables on the stack and will make lockdep
90 happy. Note also that one needs to making *sure* the completion passt to
91 work threads remains in-scope, and no references remain to on-stack data
92 when the initiating function returns.
95 Waiting for completions:
96 ------------------------
98 For a thread of execution to wait for some concurrent work to finish, it
99 calls wait_for_completion() on the initialized completion structure.
100 A typical usage scenario is:
102 structure completion setup_done;
103 init_completion(&setup_done);
104 initialze_work(...,&setup_done,...)
106 /* run non-dependent code */ /* do setup */
108 wait_for_completion(&seupt_done); complete(setup_done)
110 This is not implying any temporal order of wait_for_completion() and the
111 call to complete() - if the call to complete() happened before the call
112 to wait_for_completion() then the waiting side simply will continue
113 immediately as all dependencies are satisfied.
115 Note that wait_for_completion() is calling spin_lock_irq/spin_unlock_irq
116 so it can only be called safely when you know that interrupts are enabled.
117 Calling it from hard-irq context will result in hard to detect spurious
118 enabling of interrupts.
120 wait_for_completion():
122 void wait_for_completion(struct completion *done):
124 The default behavior is to wait without a timeout and mark the task as
125 uninterruptible. wait_for_completion() and its variants are only safe
126 in soft-interrupt or process context but not in hard-irq context.
127 As all variants of wait_for_completion() can (obviously) block for a long
128 time, you probably don't want to call this with held locks - see also
129 try_wait_for_completion() below.
135 The below variants all return status and this status should be checked in
136 most(/all) cases - in cases where the status is deliberately not checked you
137 probably want to make a note explaining this (e.g. see
138 arch/arm/kernel/smp.c:__cpu_up()).
140 A common problem that occurs is to have unclean assignment of return types,
141 so care should be taken with assigning return-values to variables of proper
142 type. Checking for the specific meaning of return values also has been found
143 to be quite inaccurate e.g. constructs like
144 if(!wait_for_completion_interruptible_timeout(...)) would execute the same
145 code path for successful completion and for the interrupted case - which is
146 probably not what you want.
148 int wait_for_completion_interruptible(struct completion *done)
150 marking the task TASK_INTERRUPTIBLE. If a signal was received while waiting.
151 It will return -ERESTARTSYS and 0 otherwise.
153 unsigned long wait_for_completion_timeout(struct completion *done,
154 unsigned long timeout)
156 The task is marked as TASK_UNINTERRUPTIBLE and will wait at most timeout
157 (in jiffies). If timeout occurs it return 0 else the remaining time in
158 jiffies (but at least 1). Timeouts are preferably passed by msecs_to_jiffies()
159 or usecs_to_jiffies(). If the returned timeout value is deliberately ignored
160 a comment should probably explain why (e.g. see drivers/mfd/wm8350-core.c
161 wm8350_read_auxadc())
163 long wait_for_completion_interruptible_timeout(
164 struct completion *done, unsigned long timeout)
166 passing a timeout in jiffies and marking the task as TASK_INTERRUPTIBLE. If a
167 signal was received it will return -ERESTARTSYS, 0 if completion timed-out and
168 the remaining time in jiffies if completion occurred.
170 Further variants include _killable which passes TASK_KILLABLE as the
171 designated tasks state and will return a -ERESTARTSYS if interrupted or
172 else 0 if completions was achieved as well as a _timeout variant.
174 long wait_for_completion_killable(struct completion *done)
175 long wait_for_completion_killable_timeout(struct completion *done,
176 unsigned long timeout)
178 The _io variants wait_for_completion_io behave the same as the non-_io
179 variants, except for accounting waiting time as waiting on IO, which has
180 an impact on how scheduling is calculated.
182 void wait_for_completion_io(struct completion *done)
183 unsigned long wait_for_completion_io_timeout(struct completion *done
184 unsigned long timeout)
187 Signaling completions:
188 ----------------------
190 A thread of execution that wants to signal that the conditions for
191 continuation have been achieved calls complete() to signal exactly one
192 of the waiters that it can continue.
194 void complete(struct completion *done)
196 or calls complete_all to signal all current and future waiters.
198 void complete_all(struct completion *done)
200 The signaling will work as expected even if completions are signaled before
201 a thread starts waiting. This is achieved by the waiter "consuming"
202 (decrementing) the done element of struct completion. Waiting threads
203 wakeup order is the same in which they were enqueued (FIFO order).
205 If complete() is called multiple times then this will allow for that number
206 of waiters to continue - each call to complete() will simply increment the
207 done element. Calling complete_all() multiple times is a bug though. Both
208 complete() and complete_all() can be called in hard-irq context safely.
210 There only can be one thread calling complete() or complete_all() on a
211 particular struct completions at any time - serialized through the wait
212 queue spinlock. Any such concurrent calls to complete() or complete_all()
213 probably are a design bug.
215 Signaling completion from hard-irq context is fine as it will appropriately
216 lock with spin_lock_irqsave/spin_unlock_irqrestore.
219 try_wait_for_completion()/completion_done():
220 --------------------------------------------
222 The try_wait_for_completion will not put the thread on the wait queue but
223 rather returns false if it would need to enqueue (block) the thread, else it
224 consumes any posted completions and returns true.
226 bool try_wait_for_completion(struct completion *done)
228 Finally to check state of a completions without changing it in any way is
229 provided by completion_done() returning false if there are any posted
230 completion that was not yet consumed by waiters implying that there are
231 waiters and true otherwise;
233 bool completion_done(struct completion *done)
235 Both try_wait_for_completion() and completion_done() are safe to be called in