[PATCH 3/5][RT] rtmutex: Revert Optimize rt lock wakeup

[Steven Rostedt]

From: Steven Rostedt <srostedt@xxxxxxxxxx>

The commit: rtmutex: Optimize rt lock wakeup

Does not do what it was suppose to do.
This is because the adaptive waiter sets its state to TASK_(UN)INTERRUPTIBLE
before going into the loop. Thus, the test in wakeup_next_waiter()
will always fail on an adaptive waiter, as it only tests to see if
the pending waiter never has its state set ot TASK_RUNNING unless
something else had woke it up.

The smp_mb() added to make this test work is just as expensive as
just calling wakeup. And since we we fail to wake up anyway, we are
doing both a smp_mb() and wakeup as well.

I tested this with dbench and we run faster without this patch.
I also tried a variant that instead fixed the loop, to change the state
only if the spinner was to go to sleep, and that still did not show
any improvement.

Cc: Gregory Haskins <ghaskins@xxxxxxxxxx>
Cc: Peter Morreale <pmorreale@xxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 kernel/rtmutex.c |   29 ++---------------------------
 1 files changed, 2 insertions(+), 27 deletions(-)

diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index 318d7ed..e218873 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -554,33 +554,8 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int savestate)
 	 */
 	if (!savestate)
 		wake_up_process(pendowner);
-	else {
-		/*
-		 * We can skip the actual (expensive) wakeup if the
-		 * waiter is already running, but we have to be careful
-		 * of race conditions because they may be about to sleep.
-		 *
-		 * The waiter-side protocol has the following pattern:
-		 * 1: Set state != RUNNING
-		 * 2: Conditionally sleep if waiter->task != NULL;
-		 *
-		 * And the owner-side has the following:
-		 * A: Set waiter->task = NULL
-		 * B: Conditionally wake if the state != RUNNING
-		 *
-		 * As long as we ensure 1->2 order, and A->B order, we
-		 * will never miss a wakeup.
-		 *
-		 * Therefore, this barrier ensures that waiter->task = NULL
-		 * is visible before we test the pendowner->state.  The
-		 * corresponding barrier is in the sleep logic.
-		 */
-		smp_mb();
-
-		/* If !RUNNING && !RUNNING_MUTEX */
-		if (pendowner->state & ~TASK_RUNNING_MUTEX)
-			wake_up_process_mutex(pendowner);
-	}
+	else
+		wake_up_process_mutex(pendowner);
 
 	rt_mutex_set_owner(lock, pendowner, RT_MUTEX_OWNER_PENDING);
 
-- 
1.7.2.3


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