linux-kernel - [PATCH tip/core/rcu 02/10] rcu: Protect rcu_boost() lockless accesses with ACCESS

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Message-Id: <1415209294-2534-2-git-send-email-paulmck@linux.vnet.ibm.com>
Date:	Wed,  5 Nov 2014 09:41:26 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...nel.org, laijs@...fujitsu.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,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	bobby.prani@...il.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 02/10] rcu: Protect rcu_boost() lockless accesses with ACCESS_ONCE()

From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>

This commit prevents random compiler optimizations by applying
ACCESS_ONCE() to lockless accesses.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.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 3ec85cb5d544..d59913ef8360 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1127,7 +1127,8 @@ static int rcu_boost(struct rcu_node *rnp)
 	struct task_struct *t;
 	struct list_head *tb;
 
-	if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL)
+	if (ACCESS_ONCE(rnp->exp_tasks) == NULL &&
+	    ACCESS_ONCE(rnp->boost_tasks) == NULL)
 		return 0;  /* Nothing left to boost. */
 
 	raw_spin_lock_irqsave(&rnp->lock, flags);
-- 
1.8.1.5

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