percpu-rw-semaphores: use rcu_read_lock_sched

Use rcu_read_lock_sched / rcu_read_unlock_sched / synchronize_sched
instead of rcu_read_lock / rcu_read_unlock / synchronize_rcu.

This is an optimization. The RCU-protected region is very small, so
there will be no latency problems if we disable preempt in this region.

So we use rcu_read_lock_sched / rcu_read_unlock_sched that translates
to preempt_disable / preempt_disable. It is smaller (and supposedly
faster) than preemptible rcu_read_lock / rcu_read_unlock.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
index 18f35b5..250a4ac 100644 (file)
--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -17,16 +17,16 @@ struct percpu_rw_semaphore {
 
 static inline void percpu_down_read(struct percpu_rw_semaphore *p)
 {
-       rcu_read_lock();
+       rcu_read_lock_sched();
        if (unlikely(p->locked)) {
-               rcu_read_unlock();
+               rcu_read_unlock_sched();
                mutex_lock(&p->mtx);
                this_cpu_inc(*p->counters);
                mutex_unlock(&p->mtx);
                return;
        }
        this_cpu_inc(*p->counters);
-       rcu_read_unlock();
+       rcu_read_unlock_sched();
        light_mb(); /* A, between read of p->locked and read of data, paired with D */
 }
 
@@ -51,7 +51,7 @@ static inline void percpu_down_write(struct percpu_rw_semaphore *p)
 {
        mutex_lock(&p->mtx);
        p->locked = true;
-       synchronize_rcu();
+       synchronize_sched(); /* make sure that all readers exit the rcu_read_lock_sched region */
        while (__percpu_count(p->counters))
                msleep(1);
        heavy_mb(); /* C, between read of p->counter and write to data, paired with B */