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+================================================
+Completions - "wait for completion" barrier APIs
+================================================
+
+Introduction:
+-------------
+
+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.
+
+Completions are a code synchronization mechanism which is preferable to any
+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 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 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;
+	};
+
+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 refer to the event that is being synchronized on.
+A good example is::
+
+	wait_for_completion(&early_console_added);
+
+	complete(&early_console_added);
+
+Good, intuitive naming (as always) helps code readability. Naming a completion
+'complete' is not helpful unless the purpose is super obvious...
+
+
+Initializing completions:
+-------------------------
+
+Dynamically allocated completion objects should preferably be embedded in data
+structures that are assured to be alive for the life-time of the function/driver,
+to prevent races with asynchronous complete() calls from occurring.
+
+Particular care should be taken when using the _timeout() or _killable()/_interruptible()
+variants of wait_for_completion(), as it must be assured that memory de-allocation
+does not happen until all related activities (complete() or reinit_completion())
+have taken place, even if these wait functions return prematurely due to a timeout
+or a signal triggering.
+
+Initializing of dynamically allocated completion objects is done via a call to
+init_completion()::
+
+	init_completion(&dynamic_object->done);
+
+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 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,
+but be aware of other races.
+
+For static declaration and initialization, macros are available.
+
+For static (or global) declarations in file scope you can use
+DECLARE_COMPLETION()::
+
+	static DECLARE_COMPLETION(setup_done);
+	DECLARE_COMPLETION(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.
+
+When a completion is declared as a local variable within a function,
+then the initialization should always use DECLARE_COMPLETION_ONSTACK()
+explicitly, not just to make lockdep happy, but also to make it clear
+that limited scope had been considered and is intentional::
+
+	DECLARE_COMPLETION_ONSTACK(setup_done)
+
+Note that when using completion objects as local variables you must be
+acutely 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 object is completely unused.
+
+To emphasise this again: in particular when using some of the waiting API variants
+with more complex outcomes, such as the timeout or signalling (_timeout(),
+_killable() and _interruptible()) variants, the wait might complete
+prematurely while the object might still be in use by another thread - and a return
+from the wait_on_completion*() caller function will deallocate the function
+stack and cause subtle data corruption if a complete() is done in some
+other thread. Simple testing might not trigger these kinds of races.
+
+If unsure, use dynamically allocated completion objects, preferably embedded
+in some other long lived object that has a boringly long life time which
+exceeds the life time of any helper threads using the completion object,
+or has a lock or other synchronization mechanism to make sure complete()
+is not called on a freed object.
+
+A naive DECLARE_COMPLETION() on the stack triggers a lockdep warning.
+
+Waiting for completions:
+------------------------
+
+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,...);
+
+	/* run non-dependent code */		/* do setup */
+
+	wait_for_completion(&setup_done);	complete(setup_done);
+
+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 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
+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 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.
+
+
+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
+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 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 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)
+
+The task is marked as TASK_UNINTERRUPTIBLE and will wait at most 'timeout'
+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)
+
+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 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/IO 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 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 field. Calling complete_all() multiple times is a bug though. Both
+complete() and complete_all() can be called in IRQ/atomic context safely.
+
+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 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
+IRQ or atomic context.