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Date:   Mon, 11 May 2020 10:29:18 -0700
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
Message-ID: <20200511172918.GW2869@paulmck-ThinkPad-P72>
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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.

							Thanx, Paul