locking/rtmutex: Optimize setting task running after being blocked

We explicitly mark the task running after returning from
a __rt_mutex_slowlock() call, which does the actual sleeping
via wait-wake-trylocking. As such, this patch does two things:

(1) refactors the code so that setting current to TASK_RUNNING
    is done by __rt_mutex_slowlock(), and not by the callers. The
    downside to this is that it becomes a bit unclear when at what
    point we block. As such I've added a comment that the task
    blocks when calling __rt_mutex_slowlock() so readers can figure
    out when it is running again.

(2) relaxes setting current's state through __set_current_state(),
    instead of it's more expensive barrier alternative. There was no
    need for the implied barrier as we're obviously not planning on
    blocking.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1422857784.18096.1.camel@stgolabs.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/rtmutex.c

@@ -1130,6 +1130,7 @@ __rt_mutex_slowlock(struct rt_mutex *lock, int state,
 		set_current_state(state);
 	}
 
+	__set_current_state(TASK_RUNNING);
 	return ret;
 }
 
@@ -1188,10 +1189,9 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,
 	ret = task_blocks_on_rt_mutex(lock, &waiter, current, chwalk);
 
 	if (likely(!ret))
+		/* sleep on the mutex */
 		ret = __rt_mutex_slowlock(lock, state, timeout, &waiter);
 
-	set_current_state(TASK_RUNNING);
-
 	if (unlikely(ret)) {
 		remove_waiter(lock, &waiter);
 		rt_mutex_handle_deadlock(ret, chwalk, &waiter);
@@ -1626,10 +1626,9 @@ int rt_mutex_finish_proxy_lock(struct rt_mutex *lock,
 
 	set_current_state(TASK_INTERRUPTIBLE);
 
+	/* sleep on the mutex */
 	ret = __rt_mutex_slowlock(lock, TASK_INTERRUPTIBLE, to, waiter);
 
-	set_current_state(TASK_RUNNING);
-
 	if (unlikely(ret))
 		remove_waiter(lock, waiter);