doc: brief user documentation for completion

Signed-off-by: Nicholas Mc Guire <der.herr@hofr.at>
[jc: cleaned up some spurious blank lines]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/scheduler/completion.txt

+completions - wait for completion handling
+==========================================
+
+This document was originally written based on 3.18.0 (linux-next)
+
+Introduction:
+-------------
+
+If you have one or more threads of execution that must wait for some process
+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_barriers and have similar use-cases.
+
+Completions are a code synchronization mechanism that 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
+kernel/sched/completion.c - for details on completion design and
+implementation see completions-design.txt
+
+
+Usage:
+------
+
+There are three parts to the 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.
+
+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:
+
+	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.
+
+Completions should be named to convey the intent of the waiter.  A good
+example is:
+
+	wait_for_completion(&early_console_added);
+
+	complete(&early_console_added);
+
+Good naming (as always) helps code readability.
+
+
+Initializing completions:
+-------------------------
+
+Initialization of dynamically allocated completions, often embedded in
+other structures, is done with:
+
+	void init_completion(&done);
+
+Initialization is accomplished by initializing the wait queue and setting
+the default state to "not available", that is, "done" is set to 0.
+
+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() on the same completions object 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.
+
+For static declaration and initialization, macros are available. These are:
+
+	static DECLARE_COMPLETION(setup_done)
+
+used for static declarations in file scope. Within functions the static
+initialization should always use:
+
+	DECLARE_COMPLETION_ONSTACK(setup_done)
+
+suitable for automatic/local variables on the stack and will make lockdep
+happy. Note also that one needs to making *sure* the completion passt to
+work threads remains in-scope, and no references remain to on-stack data
+when the initiating function returns.
+
+
+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.
+A typical usage scenario is:
+
+	structure completion setup_done;
+	init_completion(&setup_done);
+	initialze_work(...,&setup_done,...)
+
+	/* run non-dependent code */              /* do setup */
+
+	wait_for_completion(&seupt_done);         complete(setup_done)
+
+This is not implying any temporal order of 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.
+
+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 context will result in hard to detect spurious
+enabling of interrupts.
+
+wait_for_completion():
+
+	void wait_for_completion(struct completion *done):
+
+The default behavior is to wait without a timeout and mark the task as
+uninterruptible. wait_for_completion() and its variants are only safe
+in soft-interrupt or process context but not in hard-irq 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 locks - see also
+try_wait_for_completion() below.
+
+
+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
+probably want to make a note explaining this (e.g. see
+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.
+
+	int wait_for_completion_interruptible(struct completion *done)
+
+marking the task TASK_INTERRUPTIBLE. If a signal was received while waiting.
+It will return -ERESTARTSYS and 0 otherwise.
+
+	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 return 0 else the remaining time in
+jiffies (but at least 1). Timeouts are preferably passed by 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())
+
+	long wait_for_completion_interruptible_timeout(
+		struct completion *done, unsigned long timeout)
+
+passing a timeout in jiffies and marking the task as TASK_INTERRUPTIBLE. If a
+signal was received it will return -ERESTARTSYS, 0 if completion timed-out and
+the remaining time in jiffies if completion occurred.
+
+Further variants include _killable which passes TASK_KILLABLE as the
+designated tasks state and will return a -ERESTARTSYS if interrupted or
+else 0 if completions was achieved as well as a _timeout variant.
+
+	long wait_for_completion_killable(struct completion *done)
+	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 scheduling is calculated.
+
+	void wait_for_completion_io(struct completion *done)
+	unsigned long wait_for_completion_io_timeout(struct completion *done
+		unsigned long timeout)
+
+
+Signaling completions:
+----------------------
+
+A thread of execution 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.
+
+	void complete(struct completion *done)
+
+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
+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 context safely.
+
+There only can be one thread calling complete() or complete_all() on a
+particular struct completions 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
+lock with spin_lock_irqsave/spin_unlock_irqrestore.
+
+
+try_wait_for_completion()/completion_done():
+--------------------------------------------
+
+The try_wait_for_completion will not put the thread on the wait queue but
+rather returns false if it would need to enqueue (block) the thread, else it
+consumes any posted completions and returns true.
+
+     bool try_wait_for_completion(struct completion *done)
+
+Finally to check state of a completions without changing it in any way is
+provided by completion_done() returning false if there are any posted
+completion that was not yet consumed by waiters implying that there are
+waiters and true otherwise;
+
+     bool completion_done(struct completion *done)
+
+Both try_wait_for_completion() and completion_done() are safe to be called in
+hard-irq context.