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ipc/sem.c: sem preempt improve

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The strange sysv semaphore wakeup scheme has a kind of busy-wait lock
involved, which could deadlock if preemption is enabled during the "lock".

It is an implementation detail (due to a spinlock being held) that this is
actually the case.  However if "spinlocks" are made preemptible, or if the
sem lock is changed to a sleeping lock for example, then the wakeup would
become buggy.  So this might be a bugfix for -rt kernels.

Imagine waker being preempted by wakee and never clearing IN_WAKEUP -- if
wakee has higher RT priority then there is a priority inversion deadlock.
Even if there is not a priority inversion to cause a deadlock, then there
is still time wasted spinning.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Pierre Peiffer <peifferp@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Nick Piggin 2009-12-15 16:47:30 -08:00 committed by Linus Torvalds
parent 9cad200c76
commit d4212093dc
1 changed files with 23 additions and 15 deletions
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38
ipc/sem.c
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@ -398,6 +398,27 @@ undo:
return result;
}
/*
* Wake up a process waiting on the sem queue with a given error.
* The queue is invalid (may not be accessed) after the function returns.
*/
static void wake_up_sem_queue(struct sem_queue *q, int error)
{
/*
* Hold preempt off so that we don't get preempted and have the
* wakee busy-wait until we're scheduled back on. We're holding
* locks here so it may not strictly be needed, however if the
* locks become preemptible then this prevents such a problem.
*/
preempt_disable();
q->status = IN_WAKEUP;
wake_up_process(q->sleeper);
/* hands-off: q can disappear immediately after writing q->status. */
smp_wmb();
q->status = error;
preempt_enable();
}
/* Go through the pending queue for the indicated semaphore
* looking for tasks that can be completed.
*/
@ -429,17 +450,7 @@ again:
* continue.
*/
alter = q->alter;
/* wake up the waiting thread */
q->status = IN_WAKEUP;
wake_up_process(q->sleeper);
/* hands-off: q will disappear immediately after
* writing q->status.
*/
smp_wmb();
q->status = error;
wake_up_sem_queue(q, error);
if (alter)
goto again;
}
@ -523,10 +534,7 @@ static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
list_for_each_entry_safe(q, tq, &sma->sem_pending, list) {
list_del(&q->list);
q->status = IN_WAKEUP;
wake_up_process(q->sleeper); /* doesn't sleep */
smp_wmb();
q->status = -EIDRM; /* hands-off q */
wake_up_sem_queue(q, -EIDRM);
}
/* Remove the semaphore set from the IDR */