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+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_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
+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.
+
+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() twice on the same completion 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 make *sure* the completion passed to
+work threads remains in-scope, and no references remain to on-stack data
+when the initiating function returns.
+
+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().
+
+
+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:
+
+	struct completion setup_done;
+	init_completion(&setup_done);
+	initialize_work(...,&setup_done,...)
+
+	/* run non-dependent code */              /* do setup */
+
+	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
+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):
+
+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
+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.
+
+
+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)
+
+This function marks the task TASK_INTERRUPTIBLE. 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)
+
+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())
+
+	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
+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:
+
+	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 the task is accounted in scheduling stats.
+
+	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 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/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
+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 and it will never sleep.
+
+
+try_wait_for_completion()/completion_done():
+--------------------------------------------
+
+The try_wait_for_completion() function 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 one posted completion and returns true.
+
+	bool try_wait_for_completion(struct completion *done)
+
+Finally, to check the state of a completion without changing it in any way, 
+call completion_done(), which returns false if there are no posted
+completions that were 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 or atomic context.