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

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