[PATCH v4 5/8] rcu: clear defer_qs_pending at expedited IPI entry

The per-CPU clear in __note_gp_changes() (from an earlier commit)
runs only when the local CPU notices a normal-GP advance.  When an
expedited GP arrives at a CPU whose defer_qs_pending is already
PENDING, rcu_read_unlock_special() may skip irq_work queuing due to the
pending gate.

Clear defer_qs_pending on the IPI target right in rcu_exp_handler().
This makes it possible for any arming attempt that follows
the IPI within the current GP to be able to queue irq_work again,
allowing completion expedited GPs quickly than waiting for one scheduler
tick.

Signed-off-by: Joel Fernandes <[email protected]>
---
 kernel/rcu/tree_exp.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index a43469da3926..5be80d255d6a 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -763,6 +763,12 @@ static void rcu_exp_handler(void *unused)
 			 READ_ONCE(rdp->cpu_no_qs.b.exp)))
 		return;
 
+	/*
+	 * Clear defer_qs_pending so arming attempts following this IPI
+	 * within the current GP can queue irq_work again.
+	 */
+	rcu_defer_qs_clear(rdp);
+
 	/*
 	 * Second, the common case of not being in an RCU read-side
 	 * critical section.  If also enabled or idle, immediately
-- 
2.34.1