linux-kernel - Re: [PATCH] Document that wake_up(), complete() and co. imply a full memory barrier

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Date:	Wed, 22 Apr 2009 17:07:18 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Trond.Myklebust@...app.com, serue@...ibm.com, steved@...hat.com,
	viro@...iv.linux.org.uk,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Document that wake_up(), complete() and co. imply a
	full memory barrier

On 04/22, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@...hat.com> wrote:
>
> > So, I think that try_to_wake_up() implies that the LOADS after it
> > can't be reordered with STOREs before it (and wmb() of course).
>
> Note that the patch David sent says "full memory barrier", not "full
> memory barrier wrt. task->state":
>
> + (*) wake_up(), try_to_wake_up() and co. imply a full memory barrier.
> +
> + (*) complete() and co. imply a full memory barrier.
>
> These statements are not true in that form, as this code does not
> imply a full memory barrier. It does imply one on task->state
> _alone_ (and a couple of other wq-internal variables it happens to
> read for sure).
>
> But even that one isnt entirely true in the two sub-cases i noted:
> the !wq case (which can happen in object state teardown) and the
> special ->func handler (which can happen in custom wakeup code a'la
> eventpoll).
>
> So adding a comment that says "this is a full memory barrier" is
> simply not true to that extent, and is easily misunderstood. Adding
> "this is a fully memory barrier for task->state dependent data flow"
> would be more correct. (with a 'as long as wq is not NULL, and as
> long as the code using this isnt overriding ->func)
>
> Agreed?

Yes sure.

Except... not that it really matters, but the reading of ->state is
not "special". I mean,

	STORE;
	try_to_wake_up();
	LOAD;

in this case try_to_wake_up() acts as a barrier for STORE/LOAD. But
probably we should not rely on this. So personally I agree with
"for task->state dependent data flow" above.

Oleg.

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