sched/completions/Documentation: Clean up the document some more

completions - wait for completion handling

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This document was originally written based on 3.18.0 (linux-next)

Completions - "wait for completion" barrier APIs

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Introduction:

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If you have one or more threads of execution that must wait for some process

If you have one or more threads that must wait for some kernel activity

to have reached a point or a specific state, completions can provide a

race-free solution to this problem. Semantically they are somewhat like a

pthread_barrier and have similar use-cases.

pthread_barrier() and have similar use-cases.

Completions are a code synchronization mechanism which is preferable to any

misuse of locks. Any time you think of using yield() or some quirky

msleep(1) loop to allow something else to proceed, you probably want to

look into using one of the wait_for_completion*() calls instead. The

advantage of using completions is clear intent of the code, but also more

efficient code as both threads can continue until the result is actually

needed.

Completions are built on top of the generic event infrastructure in Linux,

with the event reduced to a simple flag (appropriately called "done") in

struct completion that tells the waiting threads of execution if they

can continue safely.

As completions are scheduling related, the code is found in

misuse of locks/semaphores and busy-loops. Any time you think of using

yield() or some quirky msleep(1) loop to allow something else to proceed,

you probably want to look into using one of the wait_for_completion*()

calls and complete() instead.

The advantage of using completions is that they have a well defined, focused

purpose which makes it very easy to see the intent of the code, but they

also result in more efficient code as all threads can continue execution

until the result is actually needed, and both the waiting and the signalling

is highly efficient using low level scheduler sleep/wakeup facilities.

Completions are built on top of the waitqueue and wakeup infrastructure of

the Linux scheduler. The event the threads on the waitqueue are waiting for

is reduced to a simple flag in 'struct completion', appropriately called "done".

As completions are scheduling related, the code can be found in

kernel/sched/completion.c.

Usage:

------

There are three parts to using completions, the initialization of the

struct completion, the waiting part through a call to one of the variants of

wait_for_completion() and the signaling side through a call to complete()

or complete_all(). Further there are some helper functions for checking the

state of completions.

There are three main parts to using completions:

 - the initialization of the 'struct completion' synchronization object

 - the waiting part through a call to one of the variants of wait_for_completion(),

 - the signaling side through a call to complete() or complete_all().

There are also some helper functions for checking the state of completions.

Note that while initialization must happen first, the waiting and signaling

part can happen in any order. I.e. it's entirely normal for a thread

to have marked a completion as 'done' before another thread checks whether

it has to wait for it.

To use completions one needs to include <linux/completion.h> and

create a variable of type struct completion. The structure used for

handling of completions is:

To use completions you need to #include <linux/completion.h> and

create a static or dynamic variable of type 'struct completion',

which has only two fields:

	struct completion {

		unsigned int done;

		wait_queue_head_t wait;

	};

providing the wait queue to place tasks on for waiting and the flag for

indicating the state of affairs.

This provides the ->wait waitqueue to place tasks on for waiting (if any), and

the ->done completion flag for indicating whether it's completed or not.

Completions should be named to convey the intent of the waiter. A good

example is:

Completions should be named to refer to the event that is being synchronized on.

A good example is:

	wait_for_completion(&early_console_added);

	complete(&early_console_added);

Good naming (as always) helps code readability.

Good, intuitive naming (as always) helps code readability. Naming a completion

'complete' is not helpful unless the purpose is super obvious...

Initializing completions:

-------------------------

Initialization of dynamically allocated completions, often embedded in

other structures, is done with:

Initialization of dynamically allocated completion objects, often embedded in

other structures, is done via a call to init_completion():

	void init_completion(&done);

	init_completion(&dynamic_object->done);

Initialization is accomplished by initializing the wait queue and setting

the default state to "not available", that is, "done" is set to 0.

In this call we initialize the waitqueue and set ->done to 0, i.e. "not completed"

or "not done".

The re-initialization function, reinit_completion(), simply resets the

done element to "not available", thus again to 0, without touching the

wait queue. Calling init_completion() twice on the same completion object is

->done field to 0 ("not done"), without touching the waitqueue.

Callers of this function must make sure that there are no racy

wait_for_completion() calls going on in parallel.

Calling init_completion() on the same completion object twice is

most likely a bug as it re-initializes the queue to an empty queue and

enqueued tasks could get "lost" - use reinit_completion() in that case.

enqueued tasks could get "lost" - use reinit_completion() in that case,

but be aware of other races.

For static declaration and initialization, macros are available. These are:

For static declaration and initialization, macros are available.

	static DECLARE_COMPLETION(setup_done)

For static (or global) declarations in file scope you can use DECLARE_COMPLETION():

used for static declarations in file scope. Within functions the static

initialization should always use:

	static DECLARE_COMPLETION(setup_done);

	DECLARE_COMPLETION(setup_done);

	DECLARE_COMPLETION_ONSTACK(setup_done)

Note that in this case the completion is boot time (or module load time)

initialized to 'not done' and doesn't require an init_completion() call.

suitable for automatic/local variables on the stack and will make lockdep

happy. Note also that one needs to make *sure* the completion passed to

work threads remains in-scope, and no references remain to on-stack data

when the initiating function returns.

When a completion is declared as a local variable within a function,

then the initialization should always use:

Using on-stack completions for code that calls any of the _timeout or

_interruptible/_killable variants is not advisable as they will require

additional synchronization to prevent the on-stack completion object in

the timeout/signal cases from going out of scope. Consider using dynamically

allocated completions when intending to use the _interruptible/_killable

or _timeout variants of wait_for_completion().

	DECLARE_COMPLETION_ONSTACK(setup_done)

A simple DECLARE_COMPLETION() on the stack makes lockdep unhappy.

Note that when using completion objects as local variables you must be

aware of the short life time of the function stack: the function must

not return to a calling context until all activities (such as waiting

threads) have ceased and the completion is ... completely unused.

Waiting for completions:

------------------------

For a thread of execution to wait for some concurrent work to finish, it

calls wait_for_completion() on the initialized completion structure.

For a thread to wait for some concurrent activity to finish, it

calls wait_for_completion() on the initialized completion structure:

	void wait_for_completion(struct completion *done)

A typical usage scenario is:

	CPU#1					CPU#2

	struct completion setup_done;

	init_completion(&setup_done);

	initialize_work(...,&setup_done,...)

	initialize_work(...,&setup_done,...);

	/* run non-dependent code */		/* do setup */

	wait_for_completion(&setup_done);         complete(setup_done)

	wait_for_completion(&setup_done);	complete(setup_done);

This is not implying any temporal order on wait_for_completion() and the

call to complete() - if the call to complete() happened before the call

This is not implying any particular order between wait_for_completion() and

the call to complete() - if the call to complete() happened before the call

to wait_for_completion() then the waiting side simply will continue

immediately as all dependencies are satisfied; if not, it will block until

completion is signaled by complete().

Note that wait_for_completion() is calling spin_lock_irq()/spin_unlock_irq(),

so it can only be called safely when you know that interrupts are enabled.

Calling it from hard-irq or irqs-off atomic contexts will result in

hard-to-detect spurious enabling of interrupts.

wait_for_completion():

	void wait_for_completion(struct completion *done):

Calling it from IRQs-off atomic contexts will result in hard-to-detect

spurious enabling of interrupts.

The default behavior is to wait without a timeout and to mark the task as

uninterruptible. wait_for_completion() and its variants are only safe

in process context (as they can sleep) but not in atomic context,

interrupt context, with disabled irqs, or preemption is disabled - see also

interrupt context, with disabled IRQs, or preemption is disabled - see also

try_wait_for_completion() below for handling completion in atomic/interrupt

context.

As all variants of wait_for_completion() can (obviously) block for a long

time, you probably don't want to call this with held mutexes.

time depending on the nature of the activity they are waiting for, so in

most cases you probably don't want to call this with held mutexes.

Variants available:

-------------------

wait_for_completion*() variants available:

------------------------------------------

The below variants all return status and this status should be checked in

most(/all) cases - in cases where the status is deliberately not checked you

arch/arm/kernel/smp.c:__cpu_up()).

A common problem that occurs is to have unclean assignment of return types,

so care should be taken with assigning return-values to variables of proper

type. Checking for the specific meaning of return values also has been found

to be quite inaccurate e.g. constructs like

if (!wait_for_completion_interruptible_timeout(...)) would execute the same

code path for successful completion and for the interrupted case - which is

probably not what you want.

so take care to assign return-values to variables of the proper type.

Checking for the specific meaning of return values also has been found

to be quite inaccurate, e.g. constructs like:

	if (!wait_for_completion_interruptible_timeout(...))

... would execute the same code path for successful completion and for the

interrupted case - which is probably not what you want.

	int wait_for_completion_interruptible(struct completion *done)

This function marks the task TASK_INTERRUPTIBLE. If a signal was received

while waiting it will return -ERESTARTSYS; 0 otherwise.

This function marks the task TASK_INTERRUPTIBLE while it is waiting.

If a signal was received while waiting it will return -ERESTARTSYS; 0 otherwise.

	unsigned long wait_for_completion_timeout(struct completion *done,

		unsigned long timeout)

	unsigned long wait_for_completion_timeout(struct completion *done, unsigned long timeout)

The task is marked as TASK_UNINTERRUPTIBLE and will wait at most 'timeout'

(in jiffies). If timeout occurs it returns 0 else the remaining time in

jiffies (but at least 1). Timeouts are preferably calculated with

msecs_to_jiffies() or usecs_to_jiffies(). If the returned timeout value is

deliberately ignored a comment should probably explain why (e.g. see

drivers/mfd/wm8350-core.c wm8350_read_auxadc())

jiffies. If a timeout occurs it returns 0, else the remaining time in

jiffies (but at least 1).

Timeouts are preferably calculated with msecs_to_jiffies() or usecs_to_jiffies(),

to make the code largely HZ-invariant.

If the returned timeout value is deliberately ignored a comment should probably explain

why (e.g. see drivers/mfd/wm8350-core.c wm8350_read_auxadc()).

	long wait_for_completion_interruptible_timeout(

		struct completion *done, unsigned long timeout)

	long wait_for_completion_interruptible_timeout(struct completion *done, unsigned long timeout)

This function passes a timeout in jiffies and marks the task as

TASK_INTERRUPTIBLE. If a signal was received it will return -ERESTARTSYS;

otherwise it returns 0 if the completion timed out or the remaining time in

otherwise it returns 0 if the completion timed out, or the remaining time in

jiffies if completion occurred.

Further variants include _killable which uses TASK_KILLABLE as the

designated tasks state and will return -ERESTARTSYS if it is interrupted or

else 0 if completion was achieved.  There is a _timeout variant as well:

designated tasks state and will return -ERESTARTSYS if it is interrupted,

or 0 if completion was achieved.  There is a _timeout variant as well:

	long wait_for_completion_killable(struct completion *done)

	long wait_for_completion_killable_timeout(struct completion *done,

		unsigned long timeout)

	long wait_for_completion_killable_timeout(struct completion *done, unsigned long timeout)

The _io variants wait_for_completion_io() behave the same as the non-_io

variants, except for accounting waiting time as waiting on IO, which has

an impact on how the task is accounted in scheduling stats.

variants, except for accounting waiting time as 'waiting on IO', which has

an impact on how the task is accounted in scheduling/IO stats:

	void wait_for_completion_io(struct completion *done)

	unsigned long wait_for_completion_io_timeout(struct completion *done

		unsigned long timeout)

	unsigned long wait_for_completion_io_timeout(struct completion *done, unsigned long timeout)

Signaling completions:

A thread that wants to signal that the conditions for continuation have been

achieved calls complete() to signal exactly one of the waiters that it can

continue.

continue:

	void complete(struct completion *done)

or calls complete_all() to signal all current and future waiters.

... or calls complete_all() to signal all current and future waiters:

	void complete_all(struct completion *done)

The signaling will work as expected even if completions are signaled before

a thread starts waiting. This is achieved by the waiter "consuming"

(decrementing) the done element of struct completion. Waiting threads

(decrementing) the done field of 'struct completion'. Waiting threads

wakeup order is the same in which they were enqueued (FIFO order).

If complete() is called multiple times then this will allow for that number

of waiters to continue - each call to complete() will simply increment the

done element. Calling complete_all() multiple times is a bug though. Both

complete() and complete_all() can be called in hard-irq/atomic context safely.

done field. Calling complete_all() multiple times is a bug though. Both

complete() and complete_all() can be called in IRQ/atomic context safely.

There only can be one thread calling complete() or complete_all() on a

particular struct completion at any time - serialized through the wait

There can only be one thread calling complete() or complete_all() on a

particular 'struct completion' at any time - serialized through the wait

queue spinlock. Any such concurrent calls to complete() or complete_all()

probably are a design bug.

Signaling completion from hard-irq context is fine as it will appropriately

Signaling completion from IRQ context is fine as it will appropriately

lock with spin_lock_irqsave()/spin_unlock_irqrestore() and it will never sleep.

	bool completion_done(struct completion *done)

Both try_wait_for_completion() and completion_done() are safe to be called in

hard-irq or atomic context.

IRQ or atomic context.