* Ingo Molnar <mingo@elte.hu> wrote:

quoted text
> > So I deem my fix "proven by thought experiment".  I haven't tried 
> > booting it or anything.
> 
> i actually have two fixes, made earlier today. The 'fix3' one has been 
> confirmed by Sven to fix the regression - but i think we need the 'fix
> #2' one below as well to make it complete.

i just combined them into a single fix, see below.

	Ingo

--------------------------->
Subject: semaphore: fix #3
From: Ingo Molnar <mingo@elte.hu>
Date: Sun May 11 09:51:07 CEST 2008

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/semaphore.c |   23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

Index: linux/kernel/semaphore.c
===================================================================
--- linux.orig/kernel/semaphore.c
+++ linux/kernel/semaphore.c
@@ -194,6 +194,13 @@ struct semaphore_waiter {
 	struct task_struct *task;
 };
 
+static noinline void __sched __up(struct semaphore *sem)
+{
+	struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
+						struct semaphore_waiter, list);
+	wake_up_process(waiter->task);
+}
+
 /*
  * Because this function is inlined, the 'state' parameter will be
  * constant, and thus optimised away by the compiler.  Likewise the
@@ -231,6 +238,9 @@ static inline int __sched __down_common(
 	}
 
 	list_del(&waiter.list);
+	if (unlikely(!list_empty(&sem->wait_list)) && sem->count)
+		__up(sem);
+
 	return ret;
 }
 
@@ -254,9 +264,10 @@ static noinline int __sched __down_timeo
 	return __down_common(sem, TASK_UNINTERRUPTIBLE, jiffies);
 }
 
-static noinline void __sched __up(struct semaphore *sem)
-{
-	struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
-						struct semaphore_waiter, list);
-	wake_up_process(waiter->task);
-}
+
+	/*
+	 * Rotate sleepers - to make sure all of them get woken in case
+	 * of parallel up()s:
+	 */
+	list_move_tail(&waiter->list, &sem->wait_list);
+
--
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