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[109.41.115.194]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-430f6c20781sm11952350f8f.24.2025.12.15.07.11.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 15 Dec 2025 07:11:48 -0800 (PST) Message-ID: Date: Mon, 15 Dec 2025 16:11:46 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] migration: Fix a possible crash when halting a guest during migration To: Kevin Wolf , Fabiano Rosas Cc: Stefan Hajnoczi , Peter Xu , qemu-devel@nongnu.org, qemu-block@nongnu.org, Eric Blake , Paolo Bonzini , =?UTF-8?Q?Daniel_P=2E_Berrang=C3=A9?= , Markus Armbruster , Peter Maydell , =?UTF-8?Q?Alex_Benn=C3=A9e?= References: <20251208135101.271417-1-thuth@redhat.com> <20251208144525.GA1341938@fedora> <87jyyxkna0.fsf@suse.de> <5b510f3b-796a-45fb-a63f-e87b02dace61@redhat.com> <87jyyrv1br.fsf@suse.de> From: Thomas Huth Content-Language: en-US Autocrypt: addr=thuth@redhat.com; keydata= xsFNBFH7eUwBEACzyOXKU+5Pcs6wNpKzrlJwzRl3VGZt95VCdb+FgoU9g11m7FWcOafrVRwU yYkTm9+7zBUc0sW5AuPGR/dp3pSLX/yFWsA/UB4nJsHqgDvDU7BImSeiTrnpMOTXb7Arw2a2 4CflIyFqjCpfDM4MuTmzTjXq4Uov1giGE9X6viNo1pxyEpd7PanlKNnf4PqEQp06X4IgUacW tSGj6Gcns1bCuHV8OPWLkf4hkRnu8hdL6i60Yxz4E6TqlrpxsfYwLXgEeswPHOA6Mn4Cso9O 0lewVYfFfsmokfAVMKWzOl1Sr0KGI5T9CpmRfAiSHpthhHWnECcJFwl72NTi6kUcUzG4se81 O6n9d/kTj7pzTmBdfwuOZ0YUSqcqs0W+l1NcASSYZQaDoD3/SLk+nqVeCBB4OnYOGhgmIHNW 0CwMRO/GK+20alxzk//V9GmIM2ACElbfF8+Uug3pqiHkVnKqM7W9/S1NH2qmxB6zMiJUHlTH gnVeZX0dgH27mzstcF786uPcdEqS0KJuxh2kk5IvUSL3Qn3ZgmgdxBMyCPciD/1cb7/Ahazr 3ThHQXSHXkH/aDXdfLsKVuwDzHLVSkdSnZdt5HHh75/NFHxwaTlydgfHmFFwodK8y/TjyiGZ zg2Kje38xnz8zKn9iesFBCcONXS7txENTzX0z80WKBhK+XSFJwARAQABzR5UaG9tYXMgSHV0 aCA8dGh1dGhAcmVkaGF0LmNvbT7CwXgEEwECACIFAlVgX6oCGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAAoJEC7Z13T+cC21EbIP/ii9cvT2HHGbFRl8HqGT6+7Wkb+XLMqJBMAIGiQK QIP3xk1HPTsLfVG0ao4hy/oYkGNOP8+ubLnZen6Yq3zAFiMhQ44lvgigDYJo3Ve59gfe99KX EbtB+X95ODARkq0McR6OAsPNJ7gpEUzfkQUUJTXRDQXfG/FX303Gvk+YU0spm2tsIKPl6AmV 1CegDljzjycyfJbk418MQmMu2T82kjrkEofUO2a24ed3VGC0/Uz//XCR2ZTo+vBoBUQl41BD eFFtoCSrzo3yPFS+w5fkH9NT8ChdpSlbNS32NhYQhJtr9zjWyFRf0Zk+T/1P7ECn6gTEkp5k ofFIA4MFBc/fXbaDRtBmPB0N9pqTFApIUI4vuFPPO0JDrII9dLwZ6lO9EKiwuVlvr1wwzsgq zJTPBU3qHaUO4d/8G+gD7AL/6T4zi8Jo/GmjBsnYaTzbm94lf0CjXjsOX3seMhaE6WAZOQQG tZHAO1kAPWpaxne+wtgMKthyPLNwelLf+xzGvrIKvLX6QuLoWMnWldu22z2ICVnLQChlR9d6 WW8QFEpo/FK7omuS8KvvopFcOOdlbFMM8Y/8vBgVMSsK6fsYUhruny/PahprPbYGiNIhKqz7 UvgyZVl4pBFjTaz/SbimTk210vIlkDyy1WuS8Zsn0htv4+jQPgo9rqFE4mipJjy/iboDzsFN BFH7eUwBEAC2nzfUeeI8dv0C4qrfCPze6NkryUflEut9WwHhfXCLjtvCjnoGqFelH/PE9NF4 4VPSCdvD1SSmFVzu6T9qWdcwMSaC+e7G/z0/AhBfqTeosAF5XvKQlAb9ZPkdDr7YN0a1XDfa +NgA+JZB4ROyBZFFAwNHT+HCnyzy0v9Sh3BgJJwfpXHH2l3LfncvV8rgFv0bvdr70U+On2XH 5bApOyW1WpIG5KPJlDdzcQTyptOJ1dnEHfwnABEfzI3dNf63rlxsGouX/NFRRRNqkdClQR3K gCwciaXfZ7ir7fF0u1N2UuLsWA8Ei1JrNypk+MRxhbvdQC4tyZCZ8mVDk+QOK6pyK2f4rMf/ WmqxNTtAVmNuZIwnJdjRMMSs4W4w6N/bRvpqtykSqx7VXcgqtv6eqoDZrNuhGbekQA0sAnCJ VPArerAZGArm63o39me/bRUQeQVSxEBmg66yshF9HkcUPGVeC4B0TPwz+HFcVhheo6hoJjLq knFOPLRj+0h+ZL+D0GenyqD3CyuyeTT5dGcNU9qT74bdSr20k/CklvI7S9yoQje8BeQAHtdV cvO8XCLrpGuw9SgOS7OP5oI26a0548M4KldAY+kqX6XVphEw3/6U1KTf7WxW5zYLTtadjISB X9xsRWSU+Yqs3C7oN5TIPSoj9tXMoxZkCIHWvnqGwZ7JhwARAQABwsFfBBgBAgAJBQJR+3lM AhsMAAoJEC7Z13T+cC21hPAQAIsBL9MdGpdEpvXs9CYrBkd6tS9mbaSWj6XBDfA1AEdQkBOn ZH1Qt7HJesk+qNSnLv6+jP4VwqK5AFMrKJ6IjE7jqgzGxtcZnvSjeDGPF1h2CKZQPpTw890k fy18AvgFHkVk2Oylyexw3aOBsXg6ukN44vIFqPoc+YSU0+0QIdYJp/XFsgWxnFIMYwDpxSHS 5fdDxUjsk3UBHZx+IhFjs2siVZi5wnHIqM7eK9abr2cK2weInTBwXwqVWjsXZ4tq5+jQrwDK cvxIcwXdUTLGxc4/Z/VRH1PZSvfQxdxMGmNTGaXVNfdFZjm4fz0mz+OUi6AHC4CZpwnsliGV ODqwX8Y1zic9viSTbKS01ZNp175POyWViUk9qisPZB7ypfSIVSEULrL347qY/hm9ahhqmn17 Ng255syASv3ehvX7iwWDfzXbA0/TVaqwa1YIkec+/8miicV0zMP9siRcYQkyTqSzaTFBBmqD oiT+z+/E59qj/EKfyce3sbC9XLjXv3mHMrq1tKX4G7IJGnS989E/fg6crv6NHae9Ckm7+lSs IQu4bBP2GxiRQ+NV3iV/KU3ebMRzqIC//DCOxzQNFNJAKldPe/bKZMCxEqtVoRkuJtNdp/5a yXFZ6TfE1hGKrDBYAm4vrnZ4CXFSBDllL59cFFOJCkn4Xboj/aVxxJxF30bn In-Reply-To: Content-Type: text/plain; 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=ham 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 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! 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