linux-kernel - Re: [PATCH] sched: Wake up all non-exclusive waiters in __wake_up

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Message-ID: <20170320231354.GI11100@X58A-UD3R>
Date:   Tue, 21 Mar 2017 08:13:54 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     <mingo@...nel.org>, <linux-kernel@...r.kernel.org>,
        <kernel-team@....com>
Subject: Re: [PATCH] sched: Wake up all non-exclusive waiters in
 __wake_up_common()

On Mon, Mar 20, 2017 at 11:41:47AM +0100, Peter Zijlstra wrote:
> On Wed, Mar 08, 2017 at 09:21:52AM +0900, Byungchul Park wrote:
> > __wake_up_common() should wake up all non-exclusive waiters and
> > exclusive waiters as many as nr_exclusive, but currently it does not.
> > 
> > Consider a wait queue like the following for example:
> > 
> >    A(exclusive) -> B(non-exclusive) -> C(non-exclusive)
> > 
> > Current code will wake up only A when nr_exclusive = 1, but has to wake
> > up A, B and C. Make it do as we expect.
> 
> You have the list oredered the wrong way around. We add exclusive
> waiters to the tail, therefore we'll have woken up all the !exclusive
> waiters before we start decrementing nr_exclusive.

You're right. I proposed the patch because of __add_wait_queue_exclusive(),
which queues an item at the head. But it was my mistake. Sorry.