On 12/12/2025 22.26, Fabiano Rosas wrote:
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().
As far as I can see, qmp_blockdev_del() is never called, so yes, looks like
blockdev_close_all_bdrv_states() drops the last reference here while its
still available from bdrv_next() ...
The weird thing is also that I can only reproduce the issue on s390x so far,
not on x86 ... I wonder whether we're missing something there...
Thomas
