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locking/percpu-rwsem: Add down_read_preempt_disable() 

Provide a down_read()/up_read() variant that keeps preemption disabled
over the whole thing, when possible.

This avoids a needless preemption point for constructs such as:

	percpu_down_read(&global_rwsem);
	spin_lock(&lock);
	...
	spin_unlock(&lock);
	percpu_up_read(&global_rwsem);

Which perturbs timings. In particular it was found to cure a
performance regression in a follow up patch in fs/locks.c

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
hifive-unleashed-5.1
Peter Zijlstra 2015-11-23 15:23:55 +01:00 committed by Ingo Molnar
parent 7c3f654d8e
commit 259d69b7f0
1 changed files with 18 additions and 6 deletions
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24
include/linux/percpu-rwsem.h
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@ -28,7 +28,7 @@ static struct percpu_rw_semaphore name = { \
extern int __percpu_down_read(struct percpu_rw_semaphore *, int);
extern void __percpu_up_read(struct percpu_rw_semaphore *);
static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
static inline void percpu_down_read_preempt_disable(struct percpu_rw_semaphore *sem)
{
might_sleep();
@ -46,13 +46,19 @@ static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
__this_cpu_inc(*sem->read_count);
if (unlikely(!rcu_sync_is_idle(&sem->rss)))
__percpu_down_read(sem, false); /* Unconditional memory barrier */
preempt_enable();
barrier();
/*
* The barrier() from preempt_enable() prevents the compiler from
* The barrier() prevents the compiler from
* bleeding the critical section out.
*/
}
static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
{
percpu_down_read_preempt_disable(sem);
preempt_enable();
}
static inline int percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
{
int ret = 1;
@ -76,13 +82,13 @@ static inline int percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
return ret;
}
static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
static inline void percpu_up_read_preempt_enable(struct percpu_rw_semaphore *sem)
{
/*
* The barrier() in preempt_disable() prevents the compiler from
* The barrier() prevents the compiler from
* bleeding the critical section out.
*/
preempt_disable();
barrier();
/*
* Same as in percpu_down_read().
*/
@ -95,6 +101,12 @@ static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
rwsem_release(&sem->rw_sem.dep_map, 1, _RET_IP_);
}
static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
{
preempt_disable();
percpu_up_read_preempt_enable(sem);
}
extern void percpu_down_write(struct percpu_rw_semaphore *);
extern void percpu_up_write(struct percpu_rw_semaphore *);