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Subject[PATCH tip/core/rcu 34/55] rcu: Move __rcu_read_unlock()'s barrier() within if-statement
Date
From: Paul E. McKenney <paul.mckenney@linaro.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@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.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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