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Date:   Mon, 17 Feb 2020 10:32:20 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
        tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com,
        edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com,
        joel@joelfernandes.org
Subject: Re: [PATCH tip/core/rcu 4/4] srcu: Add READ_ONCE() to srcu_struct
 ->srcu_gp_seq load
Message-ID: <20200217183220.GS2935@paulmck-ThinkPad-P72>
Reply-To: paulmck@kernel.org
References: <20200215002907.GA15895@paulmck-ThinkPad-P72>
 <20200215002932.15976-4-paulmck@kernel.org>
 <20200217124507.GT14914@hirez.programming.kicks-ass.net>
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On Mon, Feb 17, 2020 at 01:45:07PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 14, 2020 at 04:29:32PM -0800, paulmck@kernel.org wrote:
> > From: "Paul E. McKenney" <paulmck@kernel.org>
> > 
> > The load of the srcu_struct structure's ->srcu_gp_seq field in
> > srcu_funnel_gp_start() is lockless, so this commit adds the requisite
> > READ_ONCE().
> > 
> > This data race was reported by KCSAN.
> 
> But is there in actual fact a data-race? AFAICT this code was just fine.

Now that you mention it, the lock is held at that point, isn't it?  So if
that READ_ONCE() actually does anything, there is a bug somewhere else.

Good catch, I will drop this patch, thank you!

							Thanx, Paul

> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > ---
> >  kernel/rcu/srcutree.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index 119a373..90ab475 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -678,7 +678,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
> >  
> >  	/* If grace period not already done and none in progress, start it. */
> >  	if (!rcu_seq_done(&ssp->srcu_gp_seq, s) &&
> > -	    rcu_seq_state(ssp->srcu_gp_seq) == SRCU_STATE_IDLE) {
> > +	    rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)) == SRCU_STATE_IDLE) {
> >  		WARN_ON_ONCE(ULONG_CMP_GE(ssp->srcu_gp_seq, ssp->srcu_gp_seq_needed));
> >  		srcu_gp_start(ssp);
> >  		if (likely(srcu_init_done))
> > -- 
> > 2.9.5
> >