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Fix deadlock in IO pipeline

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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
This commit is contained in:
Brian Behlendorf 2018-03-16 16:46:06 -07:00 committed by GitHub
parent 17dd88352e
commit a76f3d0437
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1 changed files with 11 additions and 1 deletions
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12
module/zfs/vdev_queue.c
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@ -517,6 +517,7 @@ static zio_t *
vdev_queue_aggregate(vdev_queue_t *vq, zio_t *zio)
{
zio_t *first, *last, *aio, *dio, *mandatory, *nio;
zio_link_t *zl = NULL;
uint64_t maxgap = 0;
uint64_t size;
uint64_t limit;
@ -665,9 +666,18 @@ vdev_queue_aggregate(vdev_queue_t *vq, zio_t *zio)
zio_add_child(dio, aio);
vdev_queue_io_remove(vq, dio);
} while (dio != last);
/*
* We need to drop the vdev queue's lock to avoid a deadlock that we
* could encounter since this I/O will complete immediately.
*/
mutex_exit(&vq->vq_lock);
while ((dio = zio_walk_parents(aio, &zl)) != NULL) {
zio_vdev_io_bypass(dio);
zio_execute(dio);
} while (dio != last);
}
mutex_enter(&vq->vq_lock);
return (aio);
}