Re: [git pull] scheduler fixes

Ingo Molnar
2008-05-11T13:01:55Z

* Matthew Wilcox <matthew@wil.cx> wrote:

> + /* 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);

this needs to check for ret != 0 as well, otherwise we can be woken but
a timeout can also trigger => we lose a wakeup. I.e. like the patch
below. Hm?

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 | 24 ++++++++++++++++++------
1 file changed, 18 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 +238,10 @@ static inline int __sched __down_common(
}

list_del(&waiter.list);
+ if (unlikely(!list_empty(&sem->wait_list)) &&
+ ((sem->count > 1) || ret))
+ __up(sem);
+
return ret;
}

@@ -254,9 +265,10 @@ 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);
+