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

["Paul E. McKenney" <paulmck@kernel.org>]

On Tue, Feb 18, 2020 at 12:46:31PM +0100, Peter Zijlstra wrote:
> On Mon, Feb 17, 2020 at 03:06:57PM -0800, Paul E. McKenney wrote:
> > commit 52324a7b8a025f47a1a1a9fbd23ffe59fa764764
> > Author: Paul E. McKenney <paulmck@kernel.org>
> > Date:   Fri Jan 3 11:42:05 2020 -0800
> > 
> >     srcu: Hold srcu_struct ->lock when updating ->srcu_gp_seq
> >     
> >     A read of the srcu_struct structure's ->srcu_gp_seq field should not
> >     need READ_ONCE() when that structure's ->lock is held.  Except that this
> >     lock is not always held when updating this field.  This commit therefore
> >     acquires the lock around updates and removes a now-unneeded READ_ONCE().
> >     
> >     This data race was reported by KCSAN.
> >     
> >     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Applied, thank you!

							Thanx, Paul

> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index 119a373..c19c1df 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -450,7 +450,7 @@ static void srcu_gp_start(struct srcu_struct *ssp)
> >  	spin_unlock_rcu_node(sdp);  /* Interrupts remain disabled. */
> >  	smp_mb(); /* Order prior store to ->srcu_gp_seq_needed vs. GP start. */
> >  	rcu_seq_start(&ssp->srcu_gp_seq);
> > -	state = rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq));
> > +	state = rcu_seq_state(ssp->srcu_gp_seq);
> >  	WARN_ON_ONCE(state != SRCU_STATE_SCAN1);
> >  }
> >  
> > @@ -1130,7 +1130,9 @@ static void srcu_advance_state(struct srcu_struct *ssp)
> >  			return; /* readers present, retry later. */
> >  		}
> >  		srcu_flip(ssp);
> > +		spin_lock_irq_rcu_node(ssp);
> >  		rcu_seq_set_state(&ssp->srcu_gp_seq, SRCU_STATE_SCAN2);
> > +		spin_unlock_irq_rcu_node(ssp);
> >  	}
> >  
> >  	if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)) == SRCU_STATE_SCAN2) {