linux-kernel - [PATCH RFC tip/core/rcu 1/6] rcu: Stabilize use of num_online

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Date:	Mon, 23 Apr 2012 09:42:22 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH RFC tip/core/rcu 1/6] rcu: Stabilize use of num_online_cpus() for GP short circuit

From: "Paul E. McKenney" <paul.mckenney@...aro.org>

The rcu_blocking_is_gp() function tests to see if there is only one
online CPU, and if so, synchronize_sched() and friends become no-ops.
However, for larger systems, num_online_cpus() scans a large vector,
and might be preempted while doing so.  While preempted, any number
of CPUs might come online and go offline, potentially resulting in
num_online_cpus() returning 1 when there never had only been one
CPU online.  This would result in a too-short RCU grace period, which
could in turn result in total failure.

This commit therefore protects the num_online_cpus() with preempt_disable().

Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 include/linux/rcutree.h |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index e8ee5dd..997179f 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -101,8 +101,13 @@ extern void rcu_sched_force_quiescent_state(void);
 /* A context switch is a grace period for RCU-sched and RCU-bh. */
 static inline int rcu_blocking_is_gp(void)
 {
+	int ret;
+
 	might_sleep();  /* Check for RCU read-side critical section. */
-	return num_online_cpus() == 1;
+	preempt_disable();  /* Prevent CPU hotplug from confusing us. */
+	ret = num_online_cpus() == 1;
+	preempt_enable();
+	return ret;
 }
 
 extern void rcu_scheduler_starting(void);
-- 
1.7.8

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