On Thu, Dec 02, 2021 at 01:41:42AM +0100, Frederic Weisbecker wrote:
> On Wed, Dec 01, 2021 at 04:28:55PM -0800, Paul E. McKenney wrote:
> > Currently, rcu_advance_cbs_nowake() checks that a grace period is in
> > progress, however, that grace period could end just after the check.
> > This commit rechecks that a grace period is still in progress the lock.
>
> *while holding the node lock.
Good eyes, thank you! I will fix on next rebase.
Thanx, Paul
> > The grace period cannot end while the current CPU's rcu_node structure's
> > ->lock is held, thus avoiding false positives from the WARN_ON_ONCE().
> >
> > As Daniel Vacek noted, it is not necessary for the rcu_node structure
> > to have a CPU that has not yet passed through its quiescent state.
> >
> > Tested-By: Guillaume Morin <guillaume@xxxxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>
> Thanks!
>
> > ---
> > kernel/rcu/tree.c | 7 ++++---
> > 1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index ef8d36f580fc3..8706b30c2ac88 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1590,10 +1590,11 @@ static void __maybe_unused rcu_advance_cbs_nowake(struct rcu_node *rnp,
> > struct rcu_data *rdp)
> > {
> > rcu_lockdep_assert_cblist_protected(rdp);
> > - if (!rcu_seq_state(rcu_seq_current(&rnp->gp_seq)) ||
> > - !raw_spin_trylock_rcu_node(rnp))
> > + if (!rcu_seq_state(rcu_seq_current(&rnp->gp_seq)) || !raw_spin_trylock_rcu_node(rnp))
> > return;
> > - WARN_ON_ONCE(rcu_advance_cbs(rnp, rdp));
> > + // The grace period cannot end while we hold the rcu_node lock.
> > + if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq)))
> > + WARN_ON_ONCE(rcu_advance_cbs(rnp, rdp));
> > raw_spin_unlock_rcu_node(rnp);
> > }
> >
> > --
> > 2.31.1.189.g2e36527f23
> >
