Re: [PATCH v2 2/3] migration/block-dirty-bitmap: fix postcopy abort with bitmaps on several nodes

[Eric Blake <eblake@redhat.com>]

On Tue, Oct 06, 2026 at 07:07:37PM +0200, Denis V. Lunev wrote:
> From: Denis V. Lunev <den@openvz.org>
> 
> With dirty bitmaps on more than one node, the source switches nodes
> during the postcopy bulk phase and sends the node name again. The
> destination reads those chunks in the postcopy listen thread, which does
> not hold the BQL, and resolves the name through bdrv_lookup_bs():
> 
>   qemu-system-x86_64: ../block.c:6400: bdrv_lookup_bs: Assertion
>   `qemu_in_main_thread()' failed.
> 
> The bdrv_find_dirty_bitmap() call right after it needs the BQL or the
> dirty bitmap lock as well. Taking the BQL there would deadlock: the
> listen thread holds s->lock, and dirty_bitmap_mig_before_vm_start()
> takes s->lock with the BQL held.
> 
> Every node and bitmap that a non-START chunk refers to was set up by a
> START chunk, and those are loaded under the BQL during setup. Record the
> node alias there, and resolve all other chunks against s->bitmaps
> instead of the block graph.
> 
> Bitmaps on a single node never resend the node name after setup, which
> is why migrate-bitmaps-postcopy-test does not catch this.
> 
> Fixes: f791bf7f93f2 ("assertions for block global state API")
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Fabiano Rosas <farosas@suse.de>
> Cc: Eric Blake <eblake@redhat.com>
> Cc: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> Cc: qemu-stable@nongnu.org
> ---
>  migration/block-dirty-bitmap.c | 58 +++++++++++++++++++++++++++++++---
>  1 file changed, 54 insertions(+), 4 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>


-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org