On 15/12/2025 14.42, Kevin Wolf wrote:
Am 12.12.2025 um 22:26 hat Fabiano Rosas geschrieben:
Thomas Huth <[email protected]> writes:
On 08/12/2025 16.26, Fabiano Rosas wrote:
Stefan Hajnoczi <[email protected]> writes:
On Mon, Dec 08, 2025 at 02:51:01PM +0100, Thomas Huth wrote:
From: Thomas Huth <[email protected]>
When shutting down a guest that is currently in progress of being
migrated, there is a chance that QEMU might crash during bdrv_delete().
The backtrace looks like this:
Thread 74 "mig/src/main" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x3f7de7fc8c0 (LWP 2161436)]
0x000002aa00664012 in bdrv_delete (bs=0x2aa00f875c0) at
../../devel/qemu/block.c:5560
5560 QTAILQ_REMOVE(&graph_bdrv_states, bs, node_list);
(gdb) bt
#0 0x000002aa00664012 in bdrv_delete (bs=0x2aa00f875c0) at
../../devel/qemu/block.c:5560
#1 bdrv_unref (bs=0x2aa00f875c0) at ../../devel/qemu/block.c:7170
Backtrace stopped: Cannot access memory at address 0x3f7de7f83e0
How does the migration thread reaches here? Is this from
migration_block_inactivate()?
Unfortunately, gdb was not very helpful here (claiming that it cannot access
the memory and stack anymore), so I had to do some printf debugging. This is
what seems to happen:
Main thread: qemu_cleanup() calls migration_shutdown() -->
migration_cancel() which signals the migration thread to cancel the migration.
Migration thread: migration_thread() got kicked out the loop and calls
migration_iteration_finish(), which tries to get the BQL via bql_lock() but
that is currently held by another thread, so the migration thread is blocked
here.
Main thread: qemu_cleanup() advances to bdrv_close_all() that uses
blockdev_close_all_bdrv_states() to unref all BDS. The BDS with the name
'libvirt-1-storage' gets deleted via bdrv_delete() that way.
Has qmp_blockdev_del() ever been called to remove the BDS from the
monitor_bdrv_states list? Otherwise your debugging seems to indicate
blockdev_close_all_bdrv_states() is dropping the last reference to bs,
but it's still accessible from bdrv_next() via
bdrv_next_monitor_owned().
The reference that blockdev_close_all_bdrv_states() drops is the monitor
reference. So is this the right fix (completely untested, but matches
what qmp_blockdev_del() does)?
Kevin
diff --git a/blockdev.c b/blockdev.c
index dbd1d4d3e80..6e86c6262f9 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -686,6 +686,7 @@ void blockdev_close_all_bdrv_states(void)
GLOBAL_STATE_CODE();
QTAILQ_FOREACH_SAFE(bs, &monitor_bdrv_states, monitor_list, next_bs) {
+ QTAILQ_REMOVE(&monitor_bdrv_states, bs, monitor_list);
bdrv_unref(bs);
}
}
Thanks a lot, Kevin! This looks like the right fix for me - I gave it a try
and it fixes the crash indeed!
I also threw it into the CI and there weren't any regressions:
https://gitlab.com/thuth/qemu/-/pipelines/2215426210
https://app.travis-ci.com/github/huth/qemu/builds/277063452
Could you please send it as a proper patch?
Thanks,
Thomas
