linux-kernel - [PATCH v5 tip/core/rcu 14/16] rcu: Remove redundant preempt_disable() from rcu_note_voluntary_context

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Date:	Mon, 11 Aug 2014 15:49:03 -0700
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 v5 tip/core/rcu 14/16] rcu: Remove redundant preempt_disable() from rcu_note_voluntary_context_switch()

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

In theory, synchronize_sched() requires a read-side critical section to
order against.  In practice, preemption can be thought of as being
disabled across every machine instruction.  So this commit removes
the redundant preempt_disable() from rcu_note_voluntary_context_switch().

Reported-by: Lai Jiangshan <laijs@...fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 include/linux/rcupdate.h | 2 --
 1 file changed, 2 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index f504f797c9c8..ed6e3e2e0089 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -326,10 +326,8 @@ static inline void rcu_user_hooks_switch(struct task_struct *prev,
 extern struct srcu_struct tasks_rcu_exit_srcu;
 #define rcu_note_voluntary_context_switch(t) \
 	do { \
-		preempt_disable(); /* Exclude synchronize_sched(); */ \
 		if (ACCESS_ONCE((t)->rcu_tasks_holdout)) \
 			ACCESS_ONCE((t)->rcu_tasks_holdout) = 0; \
-		preempt_enable(); \
 	} while (0)
 #else /* #ifdef CONFIG_TASKS_RCU */
 #define TASKS_RCU(x) do { } while (0)
-- 
1.8.1.5

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