[Qemu-devel] [PATCH v2 1/2] block: Forbid bdrv_set_aio_context outside BQL

[Fam Zheng <famz@redhat.com>]

Even if the caller has the old #AioContext, there can be a deadlock, due
to the leading bdrv_drain_all:

Suppose there are three io threads (a, b, c) with each owning a BDS
(bds_a, bds_b, bds_c), and a and b want to move their own BDS to c at
the same time:

  iothread a                           iothread b
--------------------------------------------------------------------------
  bdrv_set_aio_context(bds_a, c)       bdrv_set_aio_context(bds_b, c)
  -> bdrv_drain_all()                  -> bdrv_drain_all()
     -> acquire a (OK, already has)       -> acquire a (blocked)
     -> acquire b (blocked)               -> acquire b
     -> acquire c                         -> acquire c

Current caller of bdrv_set_aio_context outside BQL is
virtio-scsi-dataplane, which will be fixed in the next patches.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@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