linux-kernel - Re: [PATCH] Documentation: Remove misleading examples of the barriers in wake

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Date:	Thu, 10 Sep 2015 19:55:57 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	Jonathan Corbet <corbet@....net>,
	Michal Hocko <mhocko@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	David Howells <dhowells@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] Documentation: Remove misleading examples of the
	barriers in wake_*()

On 09/10, Boqun Feng wrote:
>
> On Wed, Sep 09, 2015 at 12:28:22PM -0700, Paul E. McKenney wrote:
> > My feeling is
> > that we should avoid saying too much about the internals of wait_event()
> > and wake_up().

I feel the same. I simply can't understand what we are trying to
document ;)

For example,

> A STORE-LOAD barrier is implied after setting task state by wait-related functions:
>
> 	prepare_to_wait();
> 	prepare_to_wait_exclusive();
> 	prepare_to_wait_event();

I won't argue, but to me this looks misleading too.

Yes, prepare_to_wait()->set_current_state() implies mb() and thus
a STORE-LOAD barrier.

But this has nothing to do with the explanation above. We do not
need this barrier to avoid the race with wake_up(). Again, again,
we can safely rely on wq->lock and acquire/release semantics.

This barrier is only needed if you do, say,

	CONDITION = 1;

	if (waitqueue_active(wq))
		wake_up(wq);

And note that the code above is wrong without another mb() after
CONDITION = 1.

Oleg.

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