Even if the caller has both the old and the new AioContext's, there can
be a deadlock, due to the leading bdrv_drain_all.
Suppose there are four io threads (A, B, A0, B0) with A and B owning a
BDS for each (bs_a, bs_b); Now A wants to move bs_a to iothread A0, and
B wants to move bs_b to B0, at the same time:
iothread A iothread B
--------------------------------------------------------------------------
aio_context_acquire(A0) /* OK */ aio_context_acquire(B0) /* OK */
bdrv_set_aio_context(bs_a, A0) bdrv_set_aio_context(bs_b, B0)
-> bdrv_drain_all() -> bdrv_drain_all()
-> acquire A /* OK */ -> acquire A /* blocked */
-> acquire B /* blocked */ -> acquire B
... ...
Deadlock happens because A is waiting for B, and B is waiting for A.
Signed-off-by: Fam Zheng <f...@redhat.com>
Reviewed-by: Paolo Bonzini <pbonz...@redhat.com>
---
include/block/block.h | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/include/block/block.h b/include/block/block.h
index 321295e..4fce25d 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -546,8 +546,7 @@ AioContext *bdrv_get_aio_context(BlockDriverState *bs);
* Changes the #AioContext used for fd handlers, timers, and BHs by this
* BlockDriverState and all its children.
*
- * This function must be called from the old #AioContext or with a lock held so
- * the old #AioContext is not executing.
+ * This function must be called with iothread lock held.
*/
void bdrv_set_aio_context(BlockDriverState *bs, AioContext *new_context);
--
2.1.0
