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>
---
 include/linux/percpu-rwsem.h |   24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -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(stru
 	__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_trylo
 	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
 	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 *);