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Date: Thu, 10 Sep 2015 19:55:57 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        Jonathan Corbet <corbet@lwn.net>, Michal Hocko <mhocko@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        David Howells <dhowells@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] Documentation: Remove misleading examples of the
	barriers in wake_*()
Message-ID: <20150910175557.GA20640@redhat.com>
References: <1441674841-11498-1-git-send-email-boqun.feng@gmail.com> <20150909192822.GM4029@linux.vnet.ibm.com> <20150910021612.GC18494@fixme-laptop.cn.ibm.com>
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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.