test-bdrv-drain: bdrv_drain() works with cross-AioContext events

As long as nobody keeps the other I/O thread from working, there is no
reason why bdrv_drain() wouldn't work with cross-AioContext events. The
key is that the root request we're waiting for is in the AioContext
we're polling (which it always is for bdrv_drain()) so that aio_poll()
is woken up in the end.

Add a test case that shows that it works. Remove the comment in
bdrv_drain() that claims otherwise.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 0 insertions, 4 deletions

diff --git a/block/io.c b/block/io.c
index b7beaee..5f286bc 100644
--- a/block/io.c
+++ b/block/io.c
@@ -370,10 +370,6 @@ void bdrv_unapply_subtree_drain(BdrvChild *child, BlockDriverState *old_parent)
  *
  * Note that unlike bdrv_drain_all(), the caller must hold the BlockDriverState
  * AioContext.
- *
- * Only this BlockDriverState's AioContext is run, so in-flight requests must
- * not depend on events in other AioContexts.  In that case, use
- * bdrv_drain_all() instead.
  */
 void coroutine_fn bdrv_co_drain(BlockDriverState *bs)
 {