On Thu, May 08, 2008 at 05:10:28PM +0200, Ingo Molnar wrote:
quoted text
> @@ -258,7 +256,5 @@ static noinline void __sched __up(struct semaphore *sem)
>  {
>  	struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
>  						struct semaphore_waiter, list);
> -	list_del(&waiter->list);
> -	waiter->up = 1;
>  	wake_up_process(waiter->task);
>  }

This might be the problem that causes the missing wakeups.  If you have a
semaphore with n=2, and four processes calling down(), tasks A and B
acquire the semaphore and tasks C and D go to sleep.  Task A calls up()
and wakes up C.  Then task B calls up() and doesn't wake up anyone
because C hasn't run yet.  I think we need another wakeup when task C
finishes in __down_common, like this (on top of your patch):

diff --git a/kernel/semaphore.c b/kernel/semaphore.c
index 5e41217..e520ad4 100644
--- a/kernel/semaphore.c
+++ b/kernel/semaphore.c
@@ -229,6 +229,11 @@ static inline int __sched __down_common(struct semaphore *sem, long state,
 	}
 
 	list_del(&waiter.list);
+
+	/* It's possible we need to wake up the next task on the list too */
+	if (unlikely(sem->count > 1) && !list_empty(&sem->wait_list))
+		__up(sem);
+
 	return ret;
 }
 
Sven, can you try this with your workload?  I suspect this might be it
because XFS does use semaphores with n>1.

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