linux-kernel - Re: [PATCH] locking/osq_lock: fix a data race in osq_wait

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Message-ID: <20200130134851.GY14914@hirez.programming.kicks-ass.net>
Date:   Thu, 30 Jan 2020 14:48:51 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marco Elver <elver@...gle.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>, Qian Cai <cai@....pw>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [PATCH] locking/osq_lock: fix a data race in osq_wait_next

On Thu, Jan 30, 2020 at 02:39:38PM +0100, Marco Elver wrote:
> On Wed, 29 Jan 2020 at 19:40, Peter Zijlstra <peterz@...radead.org> wrote:

> > It's probably not terrible to put a READ_ONCE() there; we just need to
> > make sure the compiler doesn't do something stupid (it is known to do
> > stupid when 'volatile' is present).
> 
> Maybe we need to optimize READ_ONCE().

I think recent compilers have gotten better at volatile. In part because
of our complaints.

> 'if (data_race(..))' would also work here and has no cost.

Right, that might be the best option.