linux-kernel - [PATCH tip/core/rcu 04/12] rcu: Make expedited IPI handler return after handling critical section

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Message-Id: <20190109211850.31599-4-paulmck@linux.ibm.com>
Date:   Wed,  9 Jan 2019 13:18:42 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     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 04/12] rcu: Make expedited IPI handler return after handling critical section

During expedited RCU grace-period initialization, IPIs are sent to
all non-idle online CPUs.  The IPI handler checks to see if the CPU is
in quiescent state, reporting one if so.  This handler looks at three
different cases: (1) The CPU is not in an rcu_read_lock()-based critical
section, (2) The CPU is in the process of exiting an rcu_read_lock()-based
critical section, and (3) The CPU is in an rcu_read_lock()-based critical
section.  In case (2), execution falls through into case (3).

This is harmless from a functionality viewpoint, but can result in
needless overhead during an improbable corner case.  This commit therefore
adds the "return" statement needed to prevent fall-through.

Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
---
 kernel/rcu/tree_exp.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 928fe5893a57..6d4eb4694b6f 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -697,6 +697,7 @@ static void sync_rcu_exp_handler(void *unused)
 			WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
 		}
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+		return;
 	}

 	/*
-- 
2.17.1