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Commit d4212093 authored by Nick Piggin's avatar Nick Piggin Committed by Linus Torvalds
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ipc/sem.c: sem preempt improve 



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: default avatarNick Piggin <npiggin@suse.de>
Signed-off-by: default avatarManfred Spraul <manfred@colorfullife.com>
Cc: Pierre Peiffer <peifferp@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 9cad200c

ipc/sem.c

+23 −15
Original line number Diff line number Diff line
@@ -398,6 +398,27 @@ static int try_atomic_semop (struct sem_array * sma, struct sembuf * sops, 
	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 @@ static void update_queue (struct sem_array * sma) 
		 *   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 */