When a BlockDriverState is about to be reopened it can trigger certain
operations that need to write to disk. During this process a different
block job can be woken up. If that block job completes and also needs
to call bdrv_reopen() it can happen that it needs to do it on the same
BlockDriverState that is still in the process of being reopened.

This can have fatal consequences, like in this example:

  1) Block job A starts and sleeps after a while.
  2) Block job B starts and tries to reopen node1 (a qcow2 file).
  3) Reopening node1 means flushing and replacing its qcow2 cache.
  4) While the qcow2 cache is being flushed, job A wakes up.
  5) Job A completes and reopens node1, replacing its cache.
  6) Job B resumes, but the cache that was being flushed no longer

This patch splits the bdrv_drain_all() call to keep all block jobs
paused during bdrv_reopen_multiple(), so that step 4 can never happen
and the operation is safe.

Note that this scenario can only happen if both bdrv_reopen() calls
are made by block jobs on the same backing chain. Otherwise there's no
chance that the same BlockDriverState appears in both reopen queues.

Signed-off-by: Alberto Garcia <be...@igalia.com>
 block.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/block.c b/block.c
index 7f3e7bc..adbecd0 100644
--- a/block.c
+++ b/block.c
@@ -2090,7 +2090,7 @@ int bdrv_reopen_multiple(BlockReopenQueue *bs_queue, 
Error **errp)
     assert(bs_queue != NULL);
-    bdrv_drain_all();
+    bdrv_drain_all_begin();
     QSIMPLEQ_FOREACH(bs_entry, bs_queue, entry) {
         if (bdrv_reopen_prepare(&bs_entry->state, bs_queue, &local_err)) {
@@ -2120,6 +2120,9 @@ cleanup:
+    bdrv_drain_all_end();
     return ret;

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