rwsem: shorter spinlocked section in rwsem_down_failed_common()

This change reduces the size of the spinlocked and TASK_UNINTERRUPTIBLE
sections in rwsem_down_failed_common():

- We only need the sem->wait_lock to insert ourselves on the wait_list;
  the waiter node can be prepared outside of the wait_lock.

- The task state only needs to be set to TASK_UNINTERRUPTIBLE immediately
  before checking if we actually need to sleep; it doesn't need to protect
  the entire function.

Signed-off-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 5 deletions

diff --git a/lib/rwsem.c b/lib/rwsem.c
index 672eb33218ac..40636454cf3c 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -188,14 +188,12 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
         struct task_struct *tsk = current;
         signed long count;
 
-        set_task_state(tsk, TASK_UNINTERRUPTIBLE);
-
         /* set up my own style of waitqueue */
-        raw_spin_lock_irq(&sem->wait_lock);
         waiter.task = tsk;
         waiter.type = type;
         get_task_struct(tsk);
 
+        raw_spin_lock_irq(&sem->wait_lock);
         if (list_empty(&sem->wait_list))
                 adjustment += RWSEM_WAITING_BIAS;
         list_add_tail(&waiter.list, &sem->wait_list);
@@ -218,7 +216,8 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
         raw_spin_unlock_irq(&sem->wait_lock);
 
         /* wait to be given the lock */
-        for (;;) {
+        while (true) {
+                set_task_state(tsk, TASK_UNINTERRUPTIBLE);
                 if (!waiter.task)
                         break;
 
@@ -231,7 +230,6 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
                         }
                 raw_spin_unlock_irq(&sem->wait_lock);
                 schedule();
-                set_task_state(tsk, TASK_UNINTERRUPTIBLE);
         }
 
         tsk->state = TASK_RUNNING;