It is redundant to wake the grantee task if it is already running

Credit goes to Peter for the general idea.

Signed-off-by: Gregory Haskins <[EMAIL PROTECTED]>
Signed-off-by: Peter Morreale <[EMAIL PROTECTED]>
---

 kernel/rtmutex.c |   45 ++++++++++++++++++++++++++++++++++++++++-----
 1 files changed, 40 insertions(+), 5 deletions(-)

diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index ef52db6..bf9e230 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -531,6 +531,41 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int 
savestate)
        pendowner = waiter->task;
        waiter->task = NULL;
 
+       /*
+        * Do the wakeup before the ownership change to give any spinning
+        * waiter grantees a headstart over the other threads that will
+        * trigger once owner changes.
+        */
+       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 ((pendowner->state != TASK_RUNNING)
+                   && (pendowner->state != TASK_RUNNING_MUTEX))
+                       wake_up_process_mutex(pendowner);
+       }
+
        rt_mutex_set_owner(lock, pendowner, RT_MUTEX_OWNER_PENDING);
 
        spin_unlock(&current->pi_lock);
@@ -557,11 +592,6 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int 
savestate)
                plist_add(&next->pi_list_entry, &pendowner->pi_waiters);
        }
        spin_unlock(&pendowner->pi_lock);
-
-       if (savestate)
-               wake_up_process_mutex(pendowner);
-       else
-               wake_up_process(pendowner);
 }
 
 /*
@@ -762,6 +792,11 @@ rt_spin_lock_slowlock(struct rt_mutex *lock)
                debug_rt_mutex_print_deadlock(&waiter);
 
                update_current(TASK_UNINTERRUPTIBLE, &saved_state);
+               /*
+                * The xchg() in update_current() is an implicit barrier
+                * which we rely upon to ensure current->state is visible
+                * before we test waiter.task.
+                */
                if (waiter.task)
                        schedule_rt_mutex(lock);
                else

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