[Qemu-block] [RHEL-7.2.z qemu-kvm-rhev PATCH] virtio-blk: Fix double completion for werror=stop

[Fam Zheng]

BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1277922
Brew: http://brewweb.devel.redhat.com/brew/taskinfo?taskID=10113752

When a request R is absorbed by request M, it is appended to the
"mr_next" queue led by M, and is completed together with the completion
of M, in virtio_blk_rw_complete.

During DMA restart in virtio_blk_dma_restart_bh, requests in s->rq are
parsed and submitted again, possibly with a stale req->mr_next. It could
be a problem if the request merging in virtio_blk_handle_request hasn't
refreshed every mr_next pointer, in which case, virtio_blk_rw_complete
could walk through unexpected requests following the stale pointers.

Fix this by unsetting the pointer in virtio_blk_rw_complete. It is safe
because this req is either completed and freed right away, or it will be
restarted and parsed from scratch out of the vq later.

Signed-off-by: Fam Zheng <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>
(cherry picked from commit 10f5a72f70862d299ddbdf226d6dc71fa4ae34dd)
Signed-off-by: Fam Zheng <address@hidden>
---
 hw/block/virtio-blk.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index f9301ae..fe146ad 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -112,6 +112,10 @@ static void virtio_blk_rw_complete(void *opaque, int ret)
              * happen on the other side of the migration).
              */
             if (virtio_blk_handle_rw_error(req, -ret, is_read)) {
+                /* Break the link in case the next request is added to the
+                 * restart queue and is going to be parsed from the ring again.
+                 */
+                req->mr_next = NULL;
                 continue;
             }
         }
-- 
2.4.3