block: Only the main loop can change AioContexts

bdrv_set_aio_context_ignore() can only work in the main loop:
bdrv_drained_begin() only works in the main loop and the node's (old)
AioContext; and bdrv_drained_end() really only works in the main loop
and the node's (new) AioContext (contrary to its current comment, which
is just wrong).

Consequentially, bdrv_set_aio_context_ignore() must be called from the
main loop.  Luckily, assuming that we can make block graph changes only
from the main loop as well, all its callers do that already.

Note that changing a node's context in a sense is an operation that
changes the block graph, so it actually makes sense to require this
function to be called from the main loop.

Also, fix bdrv_drained_end()'s description.  You can only use it from
the main loop or the node's AioContext, and in the latter case, the
whole subtree must be in the same context.

Fixes: e037c09c78520cbdb6da7cfc6ad0256d5870b814
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190722133054.21781-3-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

1 files changed, 3 insertions, 5 deletions

diff --git a/include/block/block.h b/include/block/block.h
index 60f00479e0..50a07c1c33 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -667,11 +667,9 @@ void bdrv_subtree_drained_begin(BlockDriverState *bs);
  *
  * This polls @bs's AioContext until all scheduled sub-drained_ends
  * have settled.  On one hand, that may result in graph changes.  On
- * the other, this requires that all involved nodes (@bs and all of
- * its parents) are in the same AioContext, and that the caller has
- * acquired it.
- * If there are any nodes that are in different contexts from @bs,
- * these contexts must not be acquired.
+ * the other, this requires that the caller either runs in the main
+ * loop; or that all involved nodes (@bs and all of its parents) are
+ * in the caller's AioContext.
  */
 void bdrv_drained_end(BlockDriverState *bs);