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Date:   Fri, 8 May 2020 21:33:08 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Qian Cai <cai@....pw>
Cc:     Will Deacon <will@...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 Fri, May 08, 2020 at 04:59:05PM -0400, Qian Cai wrote:
> 
> 
> > On Feb 11, 2020, at 8:54 AM, Qian Cai <cai@....pw> wrote:
> > 
> > prev->next could be accessed concurrently as noticed by KCSAN,
> > 
> > write (marked) to 0xffff9d3370dbbe40 of 8 bytes by task 3294 on cpu 107:
> >  osq_lock+0x25f/0x350
> >  osq_wait_next at kernel/locking/osq_lock.c:79
> >  (inlined by) osq_lock at kernel/locking/osq_lock.c:185
> >  rwsem_optimistic_spin
> >  <snip>
> > 
> > read to 0xffff9d3370dbbe40 of 8 bytes by task 3398 on cpu 100:
> >  osq_lock+0x196/0x350
> >  osq_lock at kernel/locking/osq_lock.c:157
> >  rwsem_optimistic_spin
> >  <snip>
> > 
> > Since the write only stores NULL to prev->next and the read tests if
> > prev->next equals to this_cpu_ptr(&osq_node). Even if the value is
> > shattered, the code is still working correctly. Thus, mark it as an
> > intentional data race using the data_race() macro.
> > 
> > Signed-off-by: Qian Cai <cai@....pw>
> 
> Hmm, this patch has been dropped from linux-next from some reasons.
> 
> Paul, can you pick this up along with KCSAN fixes?
> 
> https://lore.kernel.org/lkml/1581429255-12542-1-git-send-email-cai@lca.pw/

I have queued it on -rcu, but it is too late for v5.8 via the -rcu tree,
so this is v5.9 at the earliest.  Plus I would need an ack from one of
the locking folks.

							Thanx, Paul

> > ---
> > 
> > v2: insert some code comments.
> > 
> > kernel/locking/osq_lock.c | 6 +++++-
> > 1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> > index 1f7734949ac8..f733bcd99e8a 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;
> > 
> > -- 
> > 1.8.3.1
> > 
> 

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