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 <qinw...@redhat.com>
Qing Wang has confirmed that this solves the assertion failures. Paolo/Michael: Can this still be merged for QEMU 7.1? Stefan
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