Re: [PATCH 08/14] migration: Make qemu_savevm_query_pending() available anytime

[Peter Xu <peterx@redhat.com>]

On Fri, Apr 17, 2026 at 12:26:29PM +0200, Juraj Marcin wrote:
> I agree with your approach, I was just wondering if there are other such
> races that are not handled yet. I don't think anything needs to be
> changed then, given that only save_query_pending() is susceptible to the
> race.

Emmm, irrelevant of what we're discussing.. I overlooked something here.

I just notice save_query_pending() has lock implications, this may not work
to be exported directly to a QMP handler: dirty_bitmap_state_pending(), as
one example, may take BQL inside the query function.

I added some lines into current dirty-bitmap tests, and it can already
reproduce a crash (due to deadlocking on BQL):

diff --git a/tests/qemu-iotests/tests/migrate-bitmaps-postcopy-test b/tests/qemu-iotests/tests/migrate-bitmaps-postcopy-test
index c519e6db8c..67d69d9d1e 100755
--- a/tests/qemu-iotests/tests/migrate-bitmaps-postcopy-test
+++ b/tests/qemu-iotests/tests/migrate-bitmaps-postcopy-test
@@ -162,8 +162,14 @@ class TestDirtyBitmapPostcopyMigration(iotests.QMPTestCase):
 
         self.vm_a.cmd('migrate', uri='exec:cat>' + fifo)
 
+        # Verify query-migrate working with dirty-bitmaps in precopy mode
+        self.vm_a.qmp('query-migrate')
+
         self.vm_a.cmd('migrate-start-postcopy')
 
+        # Verify query-migrate working with dirty-bitmaps in postcopy mode
+        self.vm_a.qmp('query-migrate')
+
         event_resume = self.vm_b.event_wait('RESUME')
         self.vm_b_events.append(event_resume)
         return (event_resume, discards1_sha256, all_discards_sha256)

I'll need to fix it in another way, I'll think about it.  I'll likely add
above into a separate small patch for qemu iotests to cover this case when
repost.

-- 
Peter Xu