block-backend: Queue requests while drained

This fixes devices like IDE that can still start new requests from I/O
handlers in the CPU thread while the block backend is drained.

The basic assumption is that in a drain section, no new requests should
be allowed through a BlockBackend (blk_drained_begin/end don't exist,
we get drain sections only on the node level). However, there are two
special cases where requests should not be queued:

1. Block jobs: We already make sure that block jobs are paused in a
   drain section, so they won't start new requests. However, if the
   drain_begin is called on the job's BlockBackend first, it can happen
   that we deadlock because the job stays busy until it reaches a pause
   point - which it can't if its requests aren't processed any more.

   The proper solution here would be to make all requests through the
   job's filter node instead of using a BlockBackend. For now, just
   disabling request queuing on the job BlockBackend is simpler.

2. In test cases where making requests through bdrv_* would be
   cumbersome because we'd need a BdrvChild. As we already got the
   functionality to disable request queuing from 1., use it in tests,
   too, for convenience.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/block/backup.c b/block/backup.c
index b26c22c4b8..4743c8f0bc 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -644,6 +644,7 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
     if (ret < 0) {
         goto error;
     }
+    blk_set_disable_request_queuing(job->target, true);
 
     job->on_source_error = on_source_error;
     job->on_target_error = on_target_error;