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.
> 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
>
