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Tsirkin" To: Stefan Hajnoczi Cc: qemu-devel@nongnu.org, pbonzini@redhat.com, sgarzare@redhat.com, kwolf@redhat.com, Emanuele Giuseppe Esposito , Fam Zheng , Qing Wang Subject: Re: [PATCH 7.1] virtio-scsi: fix race in virtio_scsi_dataplane_start() Message-ID: <20220805030248-mutt-send-email-mst@kernel.org> References: <20220803162824.948023-1-stefanha@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Received-SPF: pass client-ip=170.10.133.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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" On Thu, Aug 04, 2022 at 02:56:59PM -0400, Stefan Hajnoczi wrote: > On Wed, Aug 03, 2022 at 12:28:24PM -0400, Stefan Hajnoczi wrote: > > As soon as virtio_scsi_data_plane_start() attaches host notifiers the > > IOThread may start virtqueue processing. There is a race between > > IOThread virtqueue processing and virtio_scsi_data_plane_start() because > > it only assigns s->dataplane_started after attaching host notifiers. > > > > When a virtqueue handler function in the IOThread calls > > virtio_scsi_defer_to_dataplane() it may see !s->dataplane_started and > > attempt to start dataplane even though we're already in the IOThread: > > > > #0 0x00007f67b360857c __pthread_kill_implementation (libc.so.6 + 0xa257c) > > #1 0x00007f67b35bbd56 raise (libc.so.6 + 0x55d56) > > #2 0x00007f67b358e833 abort (libc.so.6 + 0x28833) > > #3 0x00007f67b358e75b __assert_fail_base.cold (libc.so.6 + 0x2875b) > > #4 0x00007f67b35b4cd6 __assert_fail (libc.so.6 + 0x4ecd6) > > #5 0x000055ca87fd411b memory_region_transaction_commit (qemu-kvm + 0x67511b) > > #6 0x000055ca87e17811 virtio_pci_ioeventfd_assign (qemu-kvm + 0x4b8811) > > #7 0x000055ca87e14836 virtio_bus_set_host_notifier (qemu-kvm + 0x4b5836) > > #8 0x000055ca87f8e14e virtio_scsi_set_host_notifier (qemu-kvm + 0x62f14e) > > #9 0x000055ca87f8dd62 virtio_scsi_dataplane_start (qemu-kvm + 0x62ed62) > > #10 0x000055ca87e14610 virtio_bus_start_ioeventfd (qemu-kvm + 0x4b5610) > > #11 0x000055ca87f8c29a virtio_scsi_handle_ctrl (qemu-kvm + 0x62d29a) > > #12 0x000055ca87fa5902 virtio_queue_host_notifier_read (qemu-kvm + 0x646902) > > #13 0x000055ca882c099e aio_dispatch_handler (qemu-kvm + 0x96199e) > > #14 0x000055ca882c1761 aio_poll (qemu-kvm + 0x962761) > > #15 0x000055ca880e1052 iothread_run (qemu-kvm + 0x782052) > > #16 0x000055ca882c562a qemu_thread_start (qemu-kvm + 0x96662a) > > > > This patch assigns s->dataplane_started before attaching host notifiers > > so that virtqueue handler functions that run in the IOThread before > > virtio_scsi_data_plane_start() returns correctly identify that dataplane > > does not need to be started. > > > > Note that s->dataplane_started does not need the AioContext lock because > > it is set before attaching host notifiers and cleared after detaching > > host notifiers. In other words, the IOThread always sees the value true > > and the main loop thread does not modify it while the IOThread is > > active. > > > > Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2099541 > > Reported-by: Qing Wang > > Qing Wang has confirmed that this solves the assertion failures. > > Paolo/Michael: Can this still be merged for QEMU 7.1? > > Stefan As a bugfix, for sure. Can I have your ack? -- MST