On Wed, 2009-01-14 at 18:18 +0100, Nick Piggin wrote:

quoted

@@ -173,21 +237,21 @@ __mutex_lock_common(struct mutex *lock, 
 			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();

Why does this need to do a preempt-disabled schedule? After we schedule
away, the next task can do arbitrary things or reschedule itself, so if
we have not anticipated such a condition here, then I can't see what
__schedule protects. At least a comment is in order?

From:
http://programming.kicks-ass.net/kernel-patches/mutex-adaptive-spin/mutex-preempt.patch

Subject: mutex: preemption fixes
From: Peter Zijlstra <redacted>
Date: Wed Jan 14 15:36:26 CET 2009

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 explicitly disabling preemption and providing a schedule() variant that
assumes preemption is already disabled.

Signed-off-by: Peter Zijlstra <redacted>

Pity to add the call overhead to schedule just for this case.

Good point, seeing any way around that?

 BTW. __schedule shouldn't need to be asmlinkage?

TBH I've no clue, probably not, Ingo?