Re: [PATCH] migration: Fix a possible crash when halting a guest during migration

[Thomas Huth <thuth@redhat.com>]

On 08/12/2025 16.26, Fabiano Rosas wrote:
> Stefan Hajnoczi <stefanha@redhat.com> writes:
> 
>> On Mon, Dec 08, 2025 at 02:51:01PM +0100, Thomas Huth wrote:
>>> From: Thomas Huth <thuth@redhat.com>
>>>
>>> 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