linux-kernel - Re: [PATCH -next v2] locking/osq_lock: annotate a data race in osq

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Message-ID: <20200511165441.GB23081@willie-the-truck>
Date:   Mon, 11 May 2020 17:54:42 +0100
From:   Will Deacon <will@...nel.org>
To:     Qian Cai <cai@....pw>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Elver Marco <elver@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [PATCH -next v2] locking/osq_lock: annotate a data race in
 osq_lock

On Mon, May 11, 2020 at 12:44:26PM -0400, Qian Cai wrote:
> 
> 
> > On May 11, 2020, at 11:58 AM, Will Deacon <will@...nel.org> wrote:
> > 
> > I'm fine with the data_race() placement, but I don't find the comment
> > very helpful. We assign the result of a READ_ONCE() to 'prev' in the
> > loop, so I don't think that the cpu_relax() is really relevant.
> > 
> > The reason we don't need READ_ONCE() here is because if we race with
> > the writer then either we'll go round the loop again after accidentally
> > thinking prev->next != node, or we'll erroneously attempt the cmpxchg()
> > because we thought they were equal and that will fail.
> > 
> > Make sense?
> 
> I think the significant concern from the previous reviews was if compilers
> could prove that prev->next == node was always true because it had no
> knowledge of the concurrency, and then took out the whole if statement
> away resulting in an infinite loop.

Hmm, I don't see how it can remove the cmpxchg(). Do you have a link to that
discussion, please?

Will