This is a note to let you know that I've just added the patch titled rcu: Tighten rcu_advance_cbs_nowake() checks to the 5.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: rcu-tighten-rcu_advance_cbs_nowake-checks.patch and it can be found in the queue-5.15 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 614ddad17f22a22e035e2ea37a04815f50362017 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" <paulmck@xxxxxxxxxx> Date: Fri, 17 Sep 2021 15:04:48 -0700 Subject: rcu: Tighten rcu_advance_cbs_nowake() checks From: Paul E. McKenney <paulmck@xxxxxxxxxx> commit 614ddad17f22a22e035e2ea37a04815f50362017 upstream. 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 while holding the rcu_node structure's 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> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- kernel/rcu/tree.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1594,10 +1594,11 @@ static void __maybe_unused rcu_advance_c 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); } Patches currently in stable-queue which might be from paulmck@xxxxxxxxxx are queue-5.15/rcu-tighten-rcu_advance_cbs_nowake-checks.patch