Subject: ipc/sem: Rework semaphore wakeups From: Peter Zijlstra Date: Wed, 14 Sep 2011 11:57:04 +0200 Subject: ipc/sem: Rework semaphore wakeups From: Peter Zijlstra 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 Suggested-by: Thomas Gleixner Reported-by: Mike Galbraith Signed-off-by: Peter Zijlstra Cc: Manfred Spraul Link: http://lkml.kernel.org/r/1315994224.5040.1.camel@twins Signed-off-by: Thomas Gleixner --- ipc/sem.c | 10 ++++++++++ 1 file changed, 10 insertions(+) Index: linux-stable/ipc/sem.c =================================================================== --- linux-stable.orig/ipc/sem.c +++ linux-stable/ipc/sem.c @@ -461,6 +461,13 @@ undo: 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 @@ -472,6 +479,7 @@ static void wake_up_sem_queue_prepare(st q->pid = error; list_add_tail(&q->simple_list, pt); +#endif } /** @@ -485,6 +493,7 @@ 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; @@ -497,6 +506,7 @@ 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)