Re: [PATCH -rt] ipc/sem: Rework semaphore wakeups

Mike Galbraith
2011-09-14T13:02:25Z

All better.

On Wed, 2011-09-14 at 11:57 +0200, Peter Zijlstra wrote:
> Subject: ipc/sem: Rework semaphore wakeups
> From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date: Tue Sep 13 15:09:40 CEST 2011
>
> Current sysv sems have a weird ass wakeup scheme that involves keeping
> preemption disabled over a potential O(n^2) loop and busy waiting on
> that on other CPUs.
>
> Kill this and simply wake the task directly from under the sem_lock.
>
> This was discovered by a migrate_disable() debug feature that
> disallows:
>
> spin_lock();
> preempt_disable();
> spin_unlock()
> preempt_enable();
>
> Cc: Manfred Spraul <manfred@colorfullife.com>
> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> Reported-by: Mike Galbraith <efault@gmx.de>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
> ipc/sem.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> Index: linux-rt/ipc/sem.c
> ===================================================================
> --- linux-rt.orig/ipc/sem.c
> +++ linux-rt/ipc/sem.c
> @@ -415,6 +415,13 @@ static int try_atomic_semop (struct sem_
> static void wake_up_sem_queue_prepare(struct list_head *pt,
> struct sem_queue *q, int error)
> {
> +#ifdef CONFIG_PREEMPT_RT_BASE
> + struct task_struct *p = q->sleeper;
> + get_task_struct(p);
> + q->status = error;
> + wake_up_process(p);
> + put_task_struct(p);
> +#else
> if (list_empty(pt)) {
> /*
> * Hold preempt off so that we don't get preempted and have the
> @@ -426,6 +433,7 @@ static void wake_up_sem_queue_prepare(st
> q->pid = error;
>
> list_add_tail(&q->simple_list, pt);
> +#endif
> }
>
> /**
> @@ -439,6 +447,7 @@ static void wake_up_sem_queue_prepare(st
> */
> static void wake_up_sem_queue_do(struct list_head *pt)
> {
> +#ifndef CONFIG_PREEMPT_RT_BASE
> struct sem_queue *q, *t;
> int did_something;
>
> @@ -451,6 +460,7 @@ static void wake_up_sem_queue_do(struct
> }
> if (did_something)
> preempt_enable();
> +#endif
> }
>
> static void unlink_queue(struct sem_array *sma, struct sem_queue *q)
>