[PATCH] spin_unlock_bh() and preempt_check_resched()

In _spin_unlock_bh(lock):
	do { \
		_raw_spin_unlock(lock); \
		preempt_enable(); \
		local_bh_enable(); \
		__release(lock); \
	} while (0)

there is no reason for using preempt_enable() instead of a simple
preempt_enable_no_resched()

Since we know bottom halves are disabled, preempt_schedule() will always
return at once (preempt_count!=0), and hence preempt_check_resched() is
useless here...

This fixes it by using "preempt_enable_no_resched()" instead of the
"preempt_enable()", and thus avoids the useless preempt_check_resched()
just before re-enabling bottom halves.
Signed-off-by: NSamuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

include/linux/spinlock.h

@@ -248,7 +248,7 @@ typedef struct {
 
 #define _spin_trylock_bh(lock)	({preempt_disable(); local_bh_disable(); \
 				_raw_spin_trylock(lock) ? \
-				1 : ({preempt_enable(); local_bh_enable(); 0;});})
+				1 : ({preempt_enable_no_resched(); local_bh_enable(); 0;});})
 
 #define _spin_lock(lock)	\
 do { \
@@ -383,7 +383,7 @@ do { \
 #define _spin_unlock_bh(lock) \
 do { \
 	_raw_spin_unlock(lock); \
-	preempt_enable(); \
+	preempt_enable_no_resched(); \
 	local_bh_enable(); \
 	__release(lock); \
 } while (0)
@@ -391,7 +391,7 @@ do { \
 #define _write_unlock_bh(lock) \
 do { \
 	_raw_write_unlock(lock); \
-	preempt_enable(); \
+	preempt_enable_no_resched(); \
 	local_bh_enable(); \
 	__release(lock); \
 } while (0)
@@ -423,8 +423,8 @@ do { \
 #define _read_unlock_bh(lock)	\
 do { \
 	_raw_read_unlock(lock);	\
+	preempt_enable_no_resched();	\
 	local_bh_enable();	\
-	preempt_enable();	\
 	__release(lock); \
 } while (0)
 

kernel/spinlock.c

@@ -294,7 +294,7 @@ EXPORT_SYMBOL(_spin_unlock_irq);
 void __lockfunc _spin_unlock_bh(spinlock_t *lock)
 {
 	_raw_spin_unlock(lock);
-	preempt_enable();
+	preempt_enable_no_resched();
 	local_bh_enable();
 }
 EXPORT_SYMBOL(_spin_unlock_bh);
@@ -318,7 +318,7 @@ EXPORT_SYMBOL(_read_unlock_irq);
 void __lockfunc _read_unlock_bh(rwlock_t *lock)
 {
 	_raw_read_unlock(lock);
-	preempt_enable();
+	preempt_enable_no_resched();
 	local_bh_enable();
 }
 EXPORT_SYMBOL(_read_unlock_bh);
@@ -342,7 +342,7 @@ EXPORT_SYMBOL(_write_unlock_irq);
 void __lockfunc _write_unlock_bh(rwlock_t *lock)
 {
 	_raw_write_unlock(lock);
-	preempt_enable();
+	preempt_enable_no_resched();
 	local_bh_enable();
 }
 EXPORT_SYMBOL(_write_unlock_bh);
@@ -354,7 +354,7 @@ int __lockfunc _spin_trylock_bh(spinlock_t *lock)
 	if (_raw_spin_trylock(lock))
 		return 1;
 
-	preempt_enable();
+	preempt_enable_no_resched();
 	local_bh_enable();
 	return 0;
 }