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From: "Paul E. McKenney" <paulmck@kernel.org>
To: Qian Cai <cai@lca.pw>
Cc: Will Deacon <will@kernel.org>, 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: Fri, 8 May 2020 21:33:08 -0700
Message-ID: <20200509043308.GL2869@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <2C8BF141-5384-475F-B713-6D473557C65C@lca.pw>

On Fri, May 08, 2020 at 04:59:05PM -0400, Qian Cai wrote:
> 
> 
> > On Feb 11, 2020, at 8:54 AM, Qian Cai <cai@lca.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@lca.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
> > 
> 

  reply index

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-11 13:54 Qian Cai
2020-05-08 20:59 ` Qian Cai
2020-05-09  4:33   ` Paul E. McKenney [this message]
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
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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