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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=thuth@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 15/12/2025 17.12, Peter Xu wrote: > On Mon, Dec 15, 2025 at 04:28:39PM +0100, Kevin Wolf wrote: >> Am 15.12.2025 um 16:11 hat Thomas Huth geschrieben: >>> On 15/12/2025 14.42, Kevin Wolf wrote: >>>> Am 12.12.2025 um 22:26 hat Fabiano Rosas geschrieben: >>>>> Thomas Huth writes: >>>>> >>>>>> On 08/12/2025 16.26, Fabiano Rosas wrote: >>>>>>> Stefan Hajnoczi writes: >>>>>>> >>>>>>>> On Mon, Dec 08, 2025 at 02:51:01PM +0100, Thomas Huth wrote: >>>>>>>>> From: Thomas Huth >>>>>>>>> >>>>>>>>> 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! >> >> Good. I think something like your patch would still be good for 11.0. >> Having undefined order in shutdown is just asking for trouble. So it >> would be good if we could be sure that migration is out of the way when >> migration_shutdown() returns. >> >> I sent the above as a proper patch to fix the immediate problem for >> 10.2. > > Thanks all! > > Does that completely fix the problem? Yes, Kevin's patch fixes the crash! > If so, IMHO we don't need the > migration change anymore until later. As replied in the other email, it > was at least intentional a few years ago (when introducing > migration_shutdown()) to not join() the migration thread here. Fine for me. Thomas