Re: [PATCH rcu 01/18] rcu: Tighten rcu_advance_cbs_nowake() checks

[Frederic Weisbecker]

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
>