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.
Migration thread: Later, migration_iteration_finish() finally gets the BQL,
and calls the migration_block_activate() function in the
MIGRATION_STATUS_CANCELLING case statement. This calls bdrv_activate_all().
bdrv_activate_all() gets a pointer to that 'libvirt-1-storage' BDS again
from bdrv_first(), and during the bdrv_next() that BDS gets unref'ed again
which is causing the crash.
==> Why is bdrv_first() still providing a BDS that have been deleted by
other threads earlier?
It sounds like the migration thread does not hold block graph refcounts
and assumes the BlockDriverStates it uses have a long enough lifetime.
I don't know the migration code well enough to say whether joining in
migration_shutdown() is okay. Another option would be expicitly holding
the necessary refcounts in the migration thread.
I agree. In principle and also because shuffling the joining around
feels like something that's prone to introduce other bugs.
I'm a little bit lost here right now ... Can you suggest a place where we
would need to increase the refcounts in the migration thread?
Thomas
