Re: [PATCH] Documentation: Remove misleading examples of the barriers in wake_*()

[Paul E. McKenney]

On Tue, Sep 08, 2015 at 09:14:01AM +0800, Boqun Feng wrote:
> Two examples for barriers in wake_up() and co. in memory-barriers.txt
> are misleading, along with their explanations:
> 
> 1.	The example which wanted to explain the write barrier in
> 	wake_up() and co. [spotted by Oleg Nesterov <oleg@xxxxxxxxxx>]
> 
> 2.	The example which wanted to explain that the write barriers in
> 	wake_up() and co. only exist iff a wakeup actually occurs.
> 
> For example #1, according to Oleg Nesterov:
> 
> >
> >       The barrier occurs before the task state is cleared
> >
> > is not actually right. This is misleading. What is really important is that
> > we have a barrier before we _read_ the task state. And again, again, the
> > fact that we actually have the write barrier is just the implementation
> > detail.
> >
> 
> And the example #2 is actually an example which could explain that the
> barriers in wait_event() and co. only exist iff a sleep actually occurs.
> 
> Further more, these barriers are only used for the correctness of
> sleeping and waking up, i.e. they exist only to guarantee the ordering
> of memory accesses to the task states and the global variables
> indicating an event. Users can't rely on them for other things, so
> memory-barriers.txt had better to call this out and remove the
> misleading examples.
> 
> This patch removes the misleading examples along with their
> explanations, calls it out that those implied barriers are only for
> sleep and wakeup related variables and adds a new example to explain the
> implied barrier in wake_up() and co.
> 
> Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>

At this point, I would favor replacing that entire section with a short
paragraph describing what guarantees are provided, perhaps with an example
showing what added barriers/locks/whatever are required.  My feeling is
that we should avoid saying too much about the internals of wait_event()
and wake_up().

Or am I missing something?

							Thanx, Paul

> ---
>  Documentation/memory-barriers.txt | 42 +++++++++++++++++----------------------
>  1 file changed, 18 insertions(+), 24 deletions(-)
> 
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index eafa6a5..07de72f 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -1948,6 +1948,10 @@ these appear to happen in the right order, the primitives to begin the process
>  of going to sleep, and the primitives to initiate a wake up imply certain
>  barriers.
> 
> +[!] Note that these implied barriers are only for the correctness of sleep and
> +wake-up. So don't rely on these barriers for things that are neither the task
> +states nor the global variables indicating the events.
> +
>  Firstly, the sleeper normally follows something like this sequence of events:
> 
>  	for (;;) {
> @@ -1997,32 +2001,22 @@ or:
>  	event_indicated = 1;
>  	wake_up_process(event_daemon);
> 
> -A write memory barrier is implied by wake_up() and co. if and only if they wake
> -something up.  The barrier occurs before the task state is cleared, and so sits
> -between the STORE to indicate the event and the STORE to set TASK_RUNNING:
> -
> -	CPU 1				CPU 2
> -	===============================	===============================
> -	set_current_state();		STORE event_indicated
> -	  smp_store_mb();		wake_up();
> -	    STORE current->state	  <write barrier>
> -	    <general barrier>		  STORE current->state
> -	LOAD event_indicated
> +A memory barrier is implied by wake_up() and co. if and only if they wake
> +something up. The memory barrier here is not necessary to be a general barrier,
> +it only needs to guarantee a STORE preceding this barrier can never be
> +reordered after a LOAD following this barrier(i.e. a STORE-LOAD barrier). This
> +barrier guarantees that the event has been indicated before the waker read the
> +wakee's task state:
> 
> -To repeat, this write memory barrier is present if and only if something
> -is actually awakened.  To see this, consider the following sequence of
> -events, where X and Y are both initially zero:
> +	CPU 1
> +	===============================
> +	STORE event_indicated;
> +	wake_up_process(wakee);
> +	  <STORE-LOAD barrier>
> +	  LOAD wakee->state;
> 
> -	CPU 1				CPU 2
> -	===============================	===============================
> -	X = 1;				STORE event_indicated
> -	smp_mb();			wake_up();
> -	Y = 1;				wait_event(wq, Y == 1);
> -	wake_up();			  load from Y sees 1, no memory barrier
> -					load from X might see 0
> -
> -In contrast, if a wakeup does occur, CPU 2's load from X would be guaranteed
> -to see 1.
> +This barrier pairs with the general barrier implied by set_current_state() on
> +the sleeper side.
> 
>  The available waker functions include:
> 
> -- 
> 2.5.1
> 

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