[Qemu-devel] [STABLE][PATCH 13/14] block: Fix early failure in multiwrite

[Kevin Wolf <kwolf@redhat.com>]

bdrv_aio_writev may call the callback immediately (and it will commonly do so
in error cases). Current code doesn't consider this. For details see the
comment added by this patch.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 453f9a1652629e5805995b165be2e634c8487139)

Conflicts:

	block.c

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block.c |   37 ++++++++++++++++++++++++++++++-------
 1 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/block.c b/block.c
index e3b3a55..80f2fae 100644
--- a/block.c
+++ b/block.c
@@ -1802,8 +1802,29 @@ int bdrv_aio_multiwrite(BlockDriverState *bs, BlockRequest *reqs, int num_reqs)
     // Check for mergable requests
     num_reqs = multiwrite_merge(bs, reqs, num_reqs, mcb);
 
-    // Run the aio requests
+    /*
+     * Run the aio requests. As soon as one request can't be submitted
+     * successfully, fail all requests that are not yet submitted (we must
+     * return failure for all requests anyway)
+     *
+     * num_requests cannot be set to the right value immediately: If
+     * bdrv_aio_writev fails for some request, num_requests would be too high
+     * and therefore multiwrite_cb() would never recognize the multiwrite
+     * request as completed. We also cannot use the loop variable i to set it
+     * when the first request fails because the callback may already have been
+     * called for previously submitted requests. Thus, num_requests must be
+     * incremented for each request that is submitted.
+     *
+     * The problem that callbacks may be called early also means that we need
+     * to take care that num_requests doesn't become 0 before all requests are
+     * submitted - multiwrite_cb() would consider the multiwrite request
+     * completed. A dummy request that is "completed" by a manual call to
+     * multiwrite_cb() takes care of this.
+     */
+    mcb->num_requests = 1;
+
     for (i = 0; i < num_reqs; i++) {
+        mcb->num_requests++;
         acb = bdrv_aio_writev(bs, reqs[i].sector, reqs[i].qiov,
             reqs[i].nb_sectors, multiwrite_cb, mcb);
 
@@ -1811,23 +1832,25 @@ int bdrv_aio_multiwrite(BlockDriverState *bs, BlockRequest *reqs, int num_reqs)
             // We can only fail the whole thing if no request has been
             // submitted yet. Otherwise we'll wait for the submitted AIOs to
             // complete and report the error in the callback.
-            if (mcb->num_requests == 0) {
-                reqs[i].error = -EIO;
+            if (i == 0) {
                 goto fail;
             } else {
-                mcb->num_requests++;
                 multiwrite_cb(mcb, -EIO);
                 break;
             }
-        } else {
-            mcb->num_requests++;
         }
     }
 
+    /* Complete the dummy request */
+    multiwrite_cb(mcb, 0);
+
     return 0;
 
 fail:
-    free(mcb);
+    for (i = 0; i < mcb->num_callbacks; i++) {
+        reqs[i].error = -EIO;
+    }
+    qemu_free(mcb);
     return -1;
 }
 
-- 
1.7.1.1