set_dirty_tracking() was assuming that the errno value set by
bdrv_create_dirty_bitmap() would not be corrupted by either
blk_get_aio_context() or aio_context_release().  Rather than
audit whether this assumption is safe, rewrite the code to just
grab the value of errno sooner.

CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>

---
v2: fix commit message typo, no code change
---
 migration/block.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/migration/block.c b/migration/block.c
index 8d79d84..ecc838a 100644
--- a/migration/block.c
+++ b/migration/block.c
@@ -350,9 +350,9 @@ static int set_dirty_tracking(void)
         aio_context_acquire(blk_get_aio_context(bmds->blk));
         bmds->dirty_bitmap = bdrv_create_dirty_bitmap(blk_bs(bmds->blk),
                                                       BLOCK_SIZE, NULL, NULL);
+        ret = -errno;
         aio_context_release(blk_get_aio_context(bmds->blk));
         if (!bmds->dirty_bitmap) {
-            ret = -errno;
             goto fail;
         }
     }
-- 
2.9.4

On 05/16/2017 03:47 PM, Eric Blake wrote:
> set_dirty_tracking() was assuming that the errno value set by
> bdrv_create_dirty_bitmap() would not be corrupted by either
> blk_get_aio_context() or aio_context_release().  Rather than
> audit whether this assumption is safe, rewrite the code to just
> grab the value of errno sooner.
> 
> CC: qemu-stable@nongnu.org
> Signed-off-by: Eric Blake <eblake@redhat.com>
> Reviewed-by: John Snow <jsnow@redhat.com>
> Reviewed-by: Juan Quintela <quintela@redhat.com>
> 

This may still be appropriate for 2.9.1, but is no longer needed on
master thanks to Paolo's work in commit 2119882c7e.

I'll have to rebase and re-send this series, one patch shorter.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org