mutex: preemption fixes

The problem is that dropping the spinlock right before schedule is a voluntary
preemption point and can cause a schedule, right after which we schedule again.

Fix this inefficiency by keeping preemption disabled until we schedule, do this
by explicity disabling preemption and providing a schedule() variant that
assumes preemption is already disabled.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 4 insertions, 1 deletions

diff --git a/kernel/mutex.c b/kernel/mutex.c
index 357c6d221efe..524ffc33dc05 100644
--- a/kernel/mutex.c
+++ b/kernel/mutex.c
@@ -131,6 +131,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
         struct mutex_waiter waiter;
         unsigned long flags;
 
+        preempt_disable();
         spin_lock_mutex(&lock->wait_lock, flags);
 
         debug_mutex_lock_common(lock, &waiter);
@@ -170,13 +171,14 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
                         spin_unlock_mutex(&lock->wait_lock, flags);
 
                         debug_mutex_free_waiter(&waiter);
+                        preempt_enable();
                         return -EINTR;
                 }
                 __set_task_state(task, state);
 
                 /* didnt get the lock, go to sleep: */
                 spin_unlock_mutex(&lock->wait_lock, flags);
-                schedule();
+                __schedule();
                 spin_lock_mutex(&lock->wait_lock, flags);
         }
 
@@ -193,6 +195,7 @@ done:
         spin_unlock_mutex(&lock->wait_lock, flags);
 
         debug_mutex_free_waiter(&waiter);
+        preempt_enable();
 
         return 0;
 }