Re: [git pull] scheduler fixes

Matthew Wilcox
2008-05-11T13:06:31Z

On Sun, May 11, 2008 at 03:01:27PM +0200, Ingo Molnar wrote:
> * 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?

Still mangled ... and I don't see how we lose a wakeup. We test for
having the semaphore before we check for having been interrupted, and we
hold the lock the whole time.

IOW, what I think you're checking for is:

task A task B
if sem->count >0
break;
sem->count++
wake_up_process(B)
if (state == TASK_INTERRUPTIBLE && signal_pending(task))
break;

which can't happen because of sem->lock.

> 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);
> +

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