[PATCH v4] doc: brief user documentation for completion

[PATCH v4] doc: brief user documentation for completion

[Nicholas Mc Guire]

Signed-off-by: Nicholas Mc Guire <der.herr@hofr.at>
---

v3: cleanups and merged review notes from Jonathan Corbet <corbet@lwn.net>
v4: english cleanup and an important ommission on scope from
    Valdis Kletniek <Valdis.Kletnieks@vt.edu>

 Documentation/scheduler/completion.txt |  245 ++++++++++++++++++++++++++++++++
 1 file changed, 245 insertions(+)
 create mode 100644 Documentation/scheduler/completion.txt

diff --git a/Documentation/scheduler/completion.txt b/Documentation/scheduler/completion.txt
new file mode 100644
index 0000000..72cbda2
--- /dev/null
+++ b/Documentation/scheduler/completion.txt
@@ -0,0 +1,245 @@
+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.
+
-- 
1.7.10.4

Re: [PATCH v4] doc: brief user documentation for completion

[Jonathan Corbet]

On Fri, 30 Jan 2015 08:01:52 +0100
Nicholas Mc Guire <der.herr@hofr.at> wrote:

> v4: english cleanup and an important ommission on scope from
>     Valdis Kletniek <Valdis.Kletnieks@vt.edu>

Looks good, applied to the docs tree.

Thanks,

jon

Re: [PATCH v4] doc: brief user documentation for completion

[Ingo Molnar]

* Nicholas Mc Guire <der.herr@hofr.at> wrote:

> Signed-off-by: Nicholas Mc Guire <der.herr@hofr.at>
> ---
> 
> v3: cleanups and merged review notes from Jonathan Corbet <corbet@lwn.net>
> v4: english cleanup and an important ommission on scope from
>     Valdis Kletniek <Valdis.Kletnieks@vt.edu>
> 
>  Documentation/scheduler/completion.txt |  245 ++++++++++++++++++++++++++++++++
>  1 file changed, 245 insertions(+)
>  create mode 100644 Documentation/scheduler/completion.txt
> 
> diff --git a/Documentation/scheduler/completion.txt b/Documentation/scheduler/completion.txt
> new file mode 100644
> index 0000000..72cbda2
> --- /dev/null
> +++ b/Documentation/scheduler/completion.txt
> @@ -0,0 +1,245 @@
> +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

s/that is/which are ?

> +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

s/code but/code, but

> +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

s/related the/related, the

> +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

s/to the using/to using

> +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

s/completions object/completion object

> +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

s/needs to making *sure*/needs to make *sure*/
s/passt/passed ?

> +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

this does not imply any temporar order on?

> +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.

It's not just about hardirq contexts, but also about 
irqs-off atomic contexts.

> +
> +
> +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.

I don't think wait_for_completion() is safe in softirq 
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.

Not 'probably': you must not call it from atomic contexts, 
held spinlocks, elevated preempt count, disabled irqs, etc.

> +
> +
> +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

s/if(/if (/

> +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.

s/marking/This function marks

The two final sentences should probably be one?

> +
> +
> +	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

s/timeout/'timeout'

> +(in jiffies). If timeout occurs it return 0 else the remaining time in

s/it return 0/it returns 0,

> +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

s/passing/This function passes

> +signal was received it will return -ERESTARTSYS, 0 if completion timed-out and

s/timed-out/timed out

> +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

s/return a -ERESTARTSYS/return -ERESTARTSYS

> +else 0 if completions was achieved as well as a _timeout variant.

s/if completions was achieved/if completion was achieved/

> +
> +	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

s/wait_for_completion_io/wait_for_completion_io()

> +variants, except for accounting waiting time as waiting on IO, which has
> +an impact on how scheduling is calculated.

s/how scheduling is calculated/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 of execution that wants to signal that the conditions for

s/thread of execution/thread

> +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.

s/complete_all/complete_all()

> +
> +	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

s/struct completions/struct completion

> +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

s/The try_wait_for_completion will/
  The try_wait_for_completion() function will/

> +rather returns false if it would need to enqueue (block) the thread, else it

s/if it would need to enqueue/if it needed to enqueue/ ?

> +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

s/completions/completion

> +provided by completion_done() returning false if there are any posted

s/if there are any/if there is any

> +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.

irq atomic context != hard-irq context.

This needs to be fixed throughout the document.

Thanks,

	Ingo

Re: [PATCH v4] doc: brief user documentation for completion

[Nicholas Mc Guire]

On Wed, 18 Feb 2015, Ingo Molnar wrote:

> 
> * Nicholas Mc Guire <der.herr@hofr.at> wrote:
> 
> > Signed-off-by: Nicholas Mc Guire <der.herr@hofr.at>
> > ---
> > 
> > v3: cleanups and merged review notes from Jonathan Corbet <corbet@lwn.net>
> > v4: english cleanup and an important ommission on scope from
> >     Valdis Kletniek <Valdis.Kletnieks@vt.edu>
> > 
> >  Documentation/scheduler/completion.txt |  245 ++++++++++++++++++++++++++++++++
> >  1 file changed, 245 insertions(+)
> >  create mode 100644 Documentation/scheduler/completion.txt
> > 
> > diff --git a/Documentation/scheduler/completion.txt b/Documentation/scheduler/completion.txt
> > new file mode 100644
> > index 0000000..72cbda2
> > --- /dev/null
> > +++ b/Documentation/scheduler/completion.txt
> > @@ -0,0 +1,245 @@
> > +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
> 
> s/that is/which are ?
> 
> > +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
> 
> s/code but/code, but
> 
> > +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
> 
> s/related the/related, the
> 
> > +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
> 
> s/to the using/to using
> 
> > +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
> 
> s/completions object/completion object
> 
> > +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
> 
> s/needs to making *sure*/needs to make *sure*/
> s/passt/passed ?
> 
> > +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
> 
> this does not imply any temporar order on?
> 
> > +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.
> 
> It's not just about hardirq contexts, but also about 
> irqs-off atomic contexts.
>

this is what I ment with "when you know that interrupts are enabled" added in
the irqs-off atomic contexts explicitly.
 
> > +
> > +
> > +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.
> 
> I don't think wait_for_completion() is safe in softirq 
> 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.
> 
> Not 'probably': you must not call it from atomic contexts, 
> held spinlocks, elevated preempt count, disabled irqs, etc.
>
> > +
> > +
> > +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
> 
> s/if(/if (/
> 
> > +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.
> 
> s/marking/This function marks
> 
> The two final sentences should probably be one?
> 
> > +
> > +
> > +	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
> 
> s/timeout/'timeout'
> 
> > +(in jiffies). If timeout occurs it return 0 else the remaining time in
> 
> s/it return 0/it returns 0,
> 
> > +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
> 
> s/passing/This function passes
> 
> > +signal was received it will return -ERESTARTSYS, 0 if completion timed-out and
> 
> s/timed-out/timed out
> 
> > +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
> 
> s/return a -ERESTARTSYS/return -ERESTARTSYS
> 
> > +else 0 if completions was achieved as well as a _timeout variant.
> 
> s/if completions was achieved/if completion was achieved/
> 
> > +
> > +	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
> 
> s/wait_for_completion_io/wait_for_completion_io()
> 
> > +variants, except for accounting waiting time as waiting on IO, which has
> > +an impact on how scheduling is calculated.
> 
> s/how scheduling is calculated/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 of execution that wants to signal that the conditions for
> 
> s/thread of execution/thread
> 
> > +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.
> 
> s/complete_all/complete_all()
> 
> > +
> > +	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
> 
> s/struct completions/struct completion
> 
> > +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
> 
> s/The try_wait_for_completion will/
>   The try_wait_for_completion() function will/
> 
> > +rather returns false if it would need to enqueue (block) the thread, else it
> 
> s/if it would need to enqueue/if it needed to enqueue/ ?
> 
> > +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
> 
> s/completions/completion
> 
> > +provided by completion_done() returning false if there are any posted
> 
> s/if there are any/if there is any
> 
> > +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.
> 
> irq atomic context != hard-irq context.
> 
> This needs to be fixed throughout the document.
>
thanks for the detailed review notes

will give it another run regarding locking and context - I obviously need a 
rerun regarding context and locking my self.

And thanks for the language notes - atleast those are clear.

thx!
hofrat

Re: [PATCH v4] doc: brief user documentation for completion

[Jonathan Corbet]

On Thu, 19 Feb 2015 14:29:33 +0100
Nicholas Mc Guire <der.herr@hofr.at> wrote:

> thanks for the detailed review notes
> 
> will give it another run regarding locking and context - I obviously need a 
> rerun regarding context and locking my self.

Note that the document went upstream last week, so it would be best to
get changes as a patch against that version.

Thanks,

jon

Re: [PATCH v4] doc: brief user documentation for completion

[Nicholas Mc Guire]

On Thu, 19 Feb 2015, Jonathan Corbet wrote:

> On Thu, 19 Feb 2015 14:29:33 +0100
> Nicholas Mc Guire <der.herr@hofr.at> wrote:
> 
> > thanks for the detailed review notes
> > 
> > will give it another run regarding locking and context - I obviously need a 
> > rerun regarding context and locking my self.
> 
> Note that the document went upstream last week, so it would be best to
> get changes as a patch against that version.
>
for review I do think it would be best to send it out as a full
text - once the content is acked no issue with sending it out as a 
patch to the current version.

thx!
hofrat 

Re: [PATCH v4] doc: brief user documentation for completion

[Ingo Molnar]

* Nicholas Mc Guire <der.herr@hofr.at> wrote:

> On Thu, 19 Feb 2015, Jonathan Corbet wrote:
> 
> > On Thu, 19 Feb 2015 14:29:33 +0100
> > Nicholas Mc Guire <der.herr@hofr.at> wrote:
> > 
> > > thanks for the detailed review notes
> > > 
> > > will give it another run regarding locking and context - I obviously need a 
> > > rerun regarding context and locking my self.
> > 
> > Note that the document went upstream last week, so it would be best to
> > get changes as a patch against that version.
>
> for review I do think it would be best to send it out as 
> a full text - once the content is acked no issue with 
> sending it out as a patch to the current version.

That would be fine to me. You could do both: send the full 
text, attach the diff at the end.

Thanks,

	Ingo