linux-kernel - [PATCH tip/core/rcu 02/10] rcu/nocb: Never downgrade ->nocb_defer_wakeup in wake_nocb_gp

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Date:   Thu,  1 Aug 2019 16:16:27 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: [PATCH tip/core/rcu 02/10] rcu/nocb: Never downgrade ->nocb_defer_wakeup in wake_nocb_gp_defer()

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@...ux.ibm.com>
---
 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