linux-kernel - [PATCH tip/core/rcu 34/55] rcu: Move __rcu_read

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Message-Id: <1315332049-2604-34-git-send-email-paulmck@linux.vnet.ibm.com>
Date:	Tue,  6 Sep 2011 11:00:28 -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,
	patches@...aro.org, "Paul E. McKenney" <paul.mckenney@...aro.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 34/55] rcu: Move __rcu_read_unlock()'s barrier() within if-statement

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

We only need to constrain the compiler if we are actually exiting
the top-level RCU read-side critical section.  This commit therefore
moves the first barrier() cal in __rcu_read_unlock() to inside the
"if" statement, thus avoiding needless register flushes for inner
rcu_read_unlock() calls.

Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 include/linux/rcupdate.h |   14 ++------------
 kernel/rcutree_plugin.h  |    2 +-
 2 files changed, 3 insertions(+), 13 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 9873040..e86bc28 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -134,16 +134,6 @@ extern void call_rcu_sched(struct rcu_head *head,
 
 extern void synchronize_sched(void);
 
-static inline void __rcu_read_lock_bh(void)
-{
-	local_bh_disable();
-}
-
-static inline void __rcu_read_unlock_bh(void)
-{
-	local_bh_enable();
-}
-
 #ifdef CONFIG_PREEMPT_RCU
 
 extern void __rcu_read_lock(void);
@@ -686,7 +676,7 @@ static inline void rcu_read_unlock(void)
  */
 static inline void rcu_read_lock_bh(void)
 {
-	__rcu_read_lock_bh();
+	local_bh_disable();
 	__acquire(RCU_BH);
 	rcu_read_acquire_bh();
 }
@@ -700,7 +690,7 @@ static inline void rcu_read_unlock_bh(void)
 {
 	rcu_read_release_bh();
 	__release(RCU_BH);
-	__rcu_read_unlock_bh();
+	local_bh_enable();
 }
 
 /**
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 4bac5a2..ed70f6b 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -415,10 +415,10 @@ void __rcu_read_unlock(void)
 {
 	struct task_struct *t = current;
 
-	barrier();  /* needed if we ever invoke rcu_read_unlock in rcutree.c */
 	if (t->rcu_read_lock_nesting != 1)
 		--t->rcu_read_lock_nesting;
 	else {
+		barrier();  /* critical section before exit code. */
 		t->rcu_read_lock_nesting = INT_MIN;
 		barrier();  /* assign before ->rcu_read_unlock_special load */
 		if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
-- 
1.7.3.2

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