[PATCH tip/core/rcu 02/10] rcu/nocb: Never downgrade ->nocb_defer_wakeup in wake_nocb_gp_defer()

[Paul E. McKenney]

Currently, wake_nocb_gp_defer() simply stores whatever waketype was
passed in, which can result in a RCU_NOCB_WAKE_FORCE being downgraded
to RCU_NOCB_WAKE, which could in turn delay callback processing.
This commit therefore adds a check so that wake_nocb_gp_defer() only
updates ->nocb_defer_wakeup when the update increases the forcefulness,
thus avoiding downgrades.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
---
 kernel/rcu/tree_plugin.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 297b38732e28..f93603ca1672 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1598,7 +1598,8 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
 {
 	if (rdp->nocb_defer_wakeup == RCU_NOCB_WAKE_NOT)
 		mod_timer(&rdp->nocb_timer, jiffies + 1);
-	WRITE_ONCE(rdp->nocb_defer_wakeup, waketype);
+	if (rdp->nocb_defer_wakeup < waketype)
+		WRITE_ONCE(rdp->nocb_defer_wakeup, waketype);
 	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, reason);
 }
 
-- 
2.17.1