Re: [PATCH tip/core/rcu 4/4] srcu: Add READ_ONCE() to srcu_struct ->srcu_gp_seq load

[Paul E. McKenney]

On Mon, Feb 17, 2020 at 01:45:07PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 14, 2020 at 04:29:32PM -0800, paulmck@xxxxxxxxxx wrote:
> > From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> > 
> > 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@xxxxxxxxxx>
> > ---
> >  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
> >