Fix deadlock in IO pipeline
In vdev_queue_aggregate() the zio_execute() bypass should not be called under the vdev queue lock. This can result in a deadlock as shown in the stack traces below. Drop the vdev queue lock then walk the parents of the aggregate IO to determine the list of component IOs to be bypassed. This can be done safely without holding the io_lock since the new aggregate IO has not yet been returned and its parents cannot change. --- THREAD 1 --- arc_read() zio_nowait() zio_vdev_io_start() vdev_queue_io() <--- mutex_enter(vq->vq_lock) vdev_queue_io_to_issue() vdev_queue_aggregate() zio_execute() zio_vdev_io_assess() zio_wait_for_children() <- mutex_enter(zio->io_lock) --- THREAD 2 --- (inverse order) arc_read() zio_change_priority() <- mutex_enter(zio->zio_lock) vdev_queue_change_io_priority() <- mutex_enter(vq->vq_lock) Reviewed-by: Tom Caputi <tcaputi@datto.com> Reviewed-by: Don Brady <don.brady@delphix.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #7307
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@ -517,6 +517,7 @@ static zio_t *
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vdev_queue_aggregate(vdev_queue_t *vq, zio_t *zio)
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{
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zio_t *first, *last, *aio, *dio, *mandatory, *nio;
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zio_link_t *zl = NULL;
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uint64_t maxgap = 0;
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uint64_t size;
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uint64_t limit;
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@ -665,9 +666,18 @@ vdev_queue_aggregate(vdev_queue_t *vq, zio_t *zio)
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zio_add_child(dio, aio);
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vdev_queue_io_remove(vq, dio);
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} while (dio != last);
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/*
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* We need to drop the vdev queue's lock to avoid a deadlock that we
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* could encounter since this I/O will complete immediately.
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*/
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mutex_exit(&vq->vq_lock);
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while ((dio = zio_walk_parents(aio, &zl)) != NULL) {
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zio_vdev_io_bypass(dio);
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zio_execute(dio);
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} while (dio != last);
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}
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mutex_enter(&vq->vq_lock);
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return (aio);
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}
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