[RFC PATCH 09/10] rcu: Advance callbacks for expedited GP completion in rcu_core()

[Puranjay Mohan <puranjay@kernel.org>]

Even when rcu_pending() triggers rcu_core(), the normal callback
advancement path through note_gp_changes() -> __note_gp_changes() bails
out when rdp->gp_seq == rnp->gp_seq (no normal GP change). Since
expedited GPs do not update rnp->gp_seq, rcu_advance_cbs() is never
called and callbacks remain stuck in RCU_WAIT_TAIL.

Add a direct callback advancement block in rcu_core() that checks for GP
completion via rcu_segcblist_nextgp() combined with
poll_state_synchronize_rcu_full(). When detected, trylock rnp and call
rcu_advance_cbs() to move completed callbacks to RCU_DONE_TAIL. Wake the
GP kthread if rcu_advance_cbs() requests a new grace period.

Uses trylock to avoid adding contention on rnp->lock. If the lock is
contended, callbacks will be advanced on the next tick.

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 kernel/rcu/tree.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 309273a37b0a..1a92b6105de5 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2853,6 +2853,22 @@ static __latent_entropy void rcu_core(void)
 	/* Update RCU state based on any recent quiescent states. */
 	rcu_check_quiescent_state(rdp);
 
+	/* Advance callbacks if an expedited GP has completed. */
+	if (!rcu_rdp_is_offloaded(rdp) &&
+	    rcu_segcblist_is_enabled(&rdp->cblist)) {
+		struct rcu_gp_oldstate gp_state;
+
+		if (rcu_segcblist_nextgp(&rdp->cblist, &gp_state) &&
+		    poll_state_synchronize_rcu_full(&gp_state)) {
+			guard(irqsave)();
+			if (raw_spin_trylock_rcu_node(rnp)) {
+				if (rcu_advance_cbs(rnp, rdp))
+					rcu_gp_kthread_wake();
+				raw_spin_unlock_rcu_node(rnp);
+			}
+		}
+	}
+
 	/* No grace period and unregistered callbacks? */
 	if (!rcu_gp_in_progress() &&
 	    rcu_segcblist_is_enabled(&rdp->cblist) && !rcu_rdp_is_offloaded(rdp)) {
-- 
2.52.0