From: "Paul E. McKenney" <paulmck@kernel.org>
To: Will Deacon <will@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 11:07:33 -0700 [thread overview]
Message-ID: <20200511180733.GA2869@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200511173412.GC23081@willie-the-truck>
On Mon, May 11, 2020 at 06:34:13PM +0100, Will Deacon wrote:
> On Mon, May 11, 2020 at 10:29:18AM -0700, Paul E. McKenney wrote:
> > On Mon, May 11, 2020 at 05:52:17PM +0100, Will Deacon wrote:
> > > 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?
> >
> > Yes, that would also work. Either have the cpu_relax() or a barrier()
> > or whatever on the one hand, or, as you say, turn the data_race() into
> > a READ_ONCE(). I personally prefer the READ_ONCE() myself, unless that
> > would undesirably suppress other KCSAN warnings.
>
> No, I mean here is the code after this patch is applied:
>
> for (;;) {
> if (data_race(prev->next) == node &&
> cmpxchg(&prev->next, node, NULL) == node)
> break;
>
> /*
> * We can only fail the cmpxchg() racing against an unlock(),
> * in which case we should observe @node->locked becomming
> * true.
> */
> if (smp_load_acquire(&node->locked))
> return true;
>
> cpu_relax();
>
> /*
> * Or we race against a concurrent unqueue()'s step-B, in which
> * case its step-C will write us a new @node->prev pointer.
> */
> prev = READ_ONCE(node->prev);
> }
>
> I'm saying that this READ_ONCE at the end of the loop should be sufficient
> to stop the compiler making value assumptions about prev->next. Do you
> agree?
Good point, and I would certainly hope so!
Thanx, Paul
next prev parent reply other threads:[~2020-05-11 18:07 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
2020-05-11 17:29 ` Paul E. McKenney
2020-05-11 17:34 ` Will Deacon
2020-05-11 18:07 ` Paul E. McKenney [this message]
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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