From: Will Deacon <will@kernel.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Qian Cai <cai@lca.pw>, Elver Marco <elver@google.com>,
LKML <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@redhat.com>,
"Peter Zijlstra (Intel)" <peterz@infradead.org>
Subject: Re: [PATCH -next v2] locking/osq_lock: annotate a data race in osq_lock
Date: Mon, 11 May 2020 17:52:17 +0100 [thread overview]
Message-ID: <20200511165216.GA23081@willie-the-truck> (raw)
In-Reply-To: <20200511164319.GV2869@paulmck-ThinkPad-P72>
On Mon, May 11, 2020 at 09:43:19AM -0700, Paul E. McKenney wrote:
> On Mon, May 11, 2020 at 04:58:13PM +0100, Will Deacon wrote:
> > On Sat, May 09, 2020 at 02:36:54PM -0700, Paul E. McKenney wrote:
> > > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> > > index 1f77349..1de006e 100644
> > > --- a/kernel/locking/osq_lock.c
> > > +++ b/kernel/locking/osq_lock.c
> > > @@ -154,7 +154,11 @@ bool osq_lock(struct optimistic_spin_queue *lock)
> > > */
> > >
> > > for (;;) {
> > > - if (prev->next == node &&
> > > + /*
> > > + * cpu_relax() below implies a compiler barrier which would
> > > + * prevent this comparison being optimized away.
> > > + */
> > > + if (data_race(prev->next) == node &&
> > > cmpxchg(&prev->next, node, NULL) == node)
> > > break;
> >
> > 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.
>
> Suppose that the compiler loaded a value that was not equal to "node".
> In that case, the cmpxchg() won't happen, so something else must force
> the compiler to do the reload in order to avoid an infinite loop, right?
> Or am I missing something here?
Then we just go round the loop and reload prev:
prev = READ_ONCE(node->prev);
which should be enough to stop the compiler, no?
Will
next prev parent reply other threads:[~2020-05-11 16:52 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-11 13:54 [PATCH -next v2] locking/osq_lock: annotate a data race in osq_lock Qian Cai
2020-05-08 20:59 ` Qian Cai
2020-05-09 4:33 ` Paul E. McKenney
2020-05-09 13:01 ` Qian Cai
2020-05-09 16:12 ` Paul E. McKenney
2020-05-09 16:53 ` Qian Cai
2020-05-09 21:36 ` Paul E. McKenney
2020-05-11 15:58 ` Will Deacon
2020-05-11 16:43 ` Paul E. McKenney
2020-05-11 16:52 ` Will Deacon [this message]
2020-05-11 17:29 ` Paul E. McKenney
2020-05-11 17:34 ` Will Deacon
2020-05-11 18:07 ` Paul E. McKenney
2020-05-11 16:44 ` Qian Cai
2020-05-11 16:54 ` Will Deacon
2020-05-11 17:10 ` Qian Cai
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