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Date: Mon, 4 Mar 2024 09:14:58 -0800
From: "Paul E. McKenney" <paulmck@kernel.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: linke li <lilinke99@qq.com>, Davidlohr Bueso <dave@stgolabs.net>,
	Josh Triplett <josh@joshtriplett.org>,
	Frederic Weisbecker <frederic@kernel.org>,
	Neeraj Upadhyay <quic_neeraju@quicinc.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Zqiang <qiang.zhang1211@gmail.com>, linux-kernel@vger.kernel.org,
	rcu@vger.kernel.org
Subject: Re: [PATCH] rcutorture: Fix
 rcu_torture_pipe_update_one()/rcu_torture_writer() data race and concurrency
 bug
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 <f3624f39-bbb1-451d-8161-8518e4108d8e@joelfernandes.org>
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On Mon, Mar 04, 2024 at 11:19:21AM -0500, Joel Fernandes wrote:
> 
> 
> On 3/4/2024 5:54 AM, linke li wrote:
> > Some changes are done to fix a data race in commit 202489101f2e ("rcutorture: Fix rcu_torture_one_read()/rcu_torture_writer() data race")
> > 
> >  {
> >  	int i;
> > 
> > -	i = rp->rtort_pipe_count;
> > +	i = READ_ONCE(rp->rtort_pipe_count);
> >  	if (i > RCU_TORTURE_PIPE_LEN)
> >  		i = RCU_TORTURE_PIPE_LEN;
> >  	atomic_inc(&rcu_torture_wcount[i]);
> > -	if (++rp->rtort_pipe_count >= RCU_TORTURE_PIPE_LEN) {
> > +	WRITE_ONCE(rp->rtort_pipe_count, i + 1);
> > +	if (rp->rtort_pipe_count >= RCU_TORTURE_PIPE_LEN) {
> >  		rp->rtort_mbtest = 0;
> >  		return true;
> >  	}
> > 
> > But ++rp->rtort_pipe_count is meant to add itself by 1, not give i+1 to
> > rp->rtort_pipe_count, because rp->rtort_pipe_count may write by
> > rcu_torture_writer() concurrently.
> > 
> > Also, rp->rtort_pipe_count in the next line should be read using
> > READ_ONCE() because of data race.
> > 
> > Signed-off-by: linke li <lilinke99@qq.com>
> > ---
> >  kernel/rcu/rcutorture.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> > index 7567ca8e743c..00059ace4fd5 100644
> > --- a/kernel/rcu/rcutorture.c
> > +++ b/kernel/rcu/rcutorture.c
> > @@ -465,8 +465,8 @@ rcu_torture_pipe_update_one(struct rcu_torture *rp)
> >  	if (i > RCU_TORTURE_PIPE_LEN)
> >  		i = RCU_TORTURE_PIPE_LEN;
> >  	atomic_inc(&rcu_torture_wcount[i]);
> > -	WRITE_ONCE(rp->rtort_pipe_count, i + 1);
> > -	if (rp->rtort_pipe_count >= RCU_TORTURE_PIPE_LEN) {
> > +	WRITE_ONCE(rp->rtort_pipe_count, rp->rtort_pipe_count + 1);
> > +	if (READ_ONCE(rp->rtort_pipe_count) >= RCU_TORTURE_PIPE_LEN) {
> 
> I want to say, I am not convinced with the patch because what's wrong with
> writing to an old index?
> 
> You win/lose the race anyway, say the CPU executed the WRITE_ONCE() a bit too
> early/late and another WRITE_ONCE() lost/won, regardless of whether you wrote
> the "incremented i" or "the increment from the latest value of pipe_count".
> 
> Anyway, a slightly related/different question:
> 
> Should that:
> WRITE_ONCE(rp->rtort_pipe_count, rp->rtort_pipe_count + 1);
> 
> Be:
> WRITE_ONCE(rp->rtort_pipe_count, READ_ONCE(rp->rtort_pipe_count) + 1);
> 
> ?

Thank you both!

At first glance, I would argue for something like this:

------------------------------------------------------------------------

static bool
rcu_torture_pipe_update_one(struct rcu_torture *rp)
{
	int i;
	struct rcu_torture_reader_check *rtrcp = READ_ONCE(rp->rtort_chkp);

	if (rtrcp) {
		WRITE_ONCE(rp->rtort_chkp, NULL);
		smp_store_release(&rtrcp->rtc_ready, 1); // Pair with smp_load_acquire().
	}
	i = READ_ONCE(rp->rtort_pipe_count) + 1;
	if (i > RCU_TORTURE_PIPE_LEN)
		i = RCU_TORTURE_PIPE_LEN;
	atomic_inc(&rcu_torture_wcount[i]);
	WRITE_ONCE(rp->rtort_pipe_count, i);
	if (i >= RCU_TORTURE_PIPE_LEN) {
		rp->rtort_mbtest = 0;
		return true;
	}
	return false;
}

------------------------------------------------------------------------

That is, move the increment to the read and replace the re-read with
the value "i" that was just written.

Thoughts?

							Thanx, Paul