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Subject[PATCH 3/3] xen-blkback: fix shutdown race
Date
Move the call to xen_blkif_put after we have freed the request,
otherwise we have a race between the release of the request and the
cleanup done in xen_blkif_free.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Matt Rushton <mrushton@amazon.com>
Cc: Matt Wilson <msw@amazon.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>
---
drivers/block/xen-blkback/blkback.c | 28 +++++++++++++++++++++-------
1 files changed, 21 insertions(+), 7 deletions(-)

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index dcfe49f..8200aa0 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -985,17 +985,31 @@ static void __end_block_io_op(struct pending_req *pending_req, int error)
* the proper response on the ring.
*/
if (atomic_dec_and_test(&pending_req->pendcnt)) {
- xen_blkbk_unmap(pending_req->blkif,
+ struct xen_blkif *blkif = pending_req->blkif;
+
+ xen_blkbk_unmap(blkif,
pending_req->segments,
pending_req->nr_pages);
- make_response(pending_req->blkif, pending_req->id,
+ make_response(blkif, pending_req->id,
pending_req->operation, pending_req->status);
- xen_blkif_put(pending_req->blkif);
- if (atomic_read(&pending_req->blkif->refcnt) <= 2) {
- if (atomic_read(&pending_req->blkif->drain))
- complete(&pending_req->blkif->drain_complete);
+ free_req(blkif, pending_req);
+ /*
+ * Make sure the request is freed before releasing blkif,
+ * or there could be a race between free_req and the
+ * cleanup done in xen_blkif_free during shutdown.
+ *
+ * NB: The fact that we might try to wake up pending_free_wq
+ * before drain_complete (in case there's a drain going on)
+ * it's not a problem with our current implementation
+ * because we can assure there's no thread waiting on
+ * pending_free_wq if there's a drain going on, but it has
+ * to be taken into account if the current model is changed.
+ */
+ xen_blkif_put(blkif);
+ if (atomic_read(&blkif->refcnt) <= 2) {
+ if (atomic_read(&blkif->drain))
+ complete(&blkif->drain_complete);
}
- free_req(pending_req->blkif, pending_req);
}
}

--
1.7.7.5 (Apple Git-26)
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