Re: [PATCH for 7.2-rc? v2 0/5] continuing efforts to fix vhost-user issues

[Stefan Hajnoczi]

On Sat, 26 Nov 2022 at 04:45, Alex Bennée <alex.bennee@linaro.org> wrote:
>
>
> Alex Bennée <alex.bennee@linaro.org> writes:
>
> > Alex Bennée <alex.bennee@linaro.org> writes:
> >
> >> Hi,
> >>
> > <snip>
> >> I can replicate some of the other failures I've been seeing in CI by
> >> running:
> >>
> >>   ../../meson/meson.py test --repeat 10 --print-errorlogs 
> >> qtest-arm/qos-test
> >>
> >> however this seems to run everything in parallel and maybe is better
> >> at exposing race conditions. Perhaps the CI system makes those races
> >> easier to hit? Unfortunately I've not been able to figure out exactly
> >> how things go wrong in the failure case.
> >>
> > <snip>
> >
> > There is a circular call - we are in vu_gpio_stop which triggers a write
> > to vhost-user which allows us to catch a disconnect event:
> >
> >   #0  vhost_dev_is_started (hdev=0x557adf80d878) at 
> > /home/alex/lsrc/qemu.git/include/hw/virtio/vhost.h:199
> >   #1  0x0000557adbe0518a in vu_gpio_stop (vdev=0x557adf80d640) at 
> > ../../hw/virtio/vhost-user-gpio.c:138
> >   #2  0x0000557adbe04d56 in vu_gpio_disconnect (dev=0x557adf80d640) at 
> > ../../hw/virtio/vhost-user-gpio.c:255
> >   #3  0x0000557adbe049bb in vu_gpio_event (opaque=0x557adf80d640, 
> > event=CHR_EVENT_CLOSED) at ../../hw/virtio/vhost-user-gpio.c:274
>
> I suspect the best choice here is to schedule the cleanup as a later
> date. Should I use the aio_bh one shots for this or maybe an rcu cleanup
> event?
>
> Paolo, any suggestions?
>
> >   #4  0x0000557adc0539ef in chr_be_event (s=0x557adea51f10, 
> > event=CHR_EVENT_CLOSED) at ../../chardev/char.c:61
> >   #5  0x0000557adc0506aa in qemu_chr_be_event (s=0x557adea51f10, 
> > event=CHR_EVENT_CLOSED) at ../../chardev/char.c:81
> >   #6  0x0000557adc04f666 in tcp_chr_disconnect_locked (chr=0x557adea51f10) 
> > at ../../chardev/char-socket.c:470
> >   #7  0x0000557adc04c81a in tcp_chr_write (chr=0x557adea51f10, 
> > buf=0x7ffe8588cce0 "\v", len=20) at ../../chardev/char-socket.c:129

Does this mean the backend closed the connection before receiving all
the vhost-user protocol messages sent by QEMU?

This looks like a backend bug. It prevents QEMU's vhost-user client
from cleanly stopping the virtqueue (vhost_virtqueue_stop()).

QEMU is still broken if it cannot handle disconnect at any time. Maybe
a simple solution for that is to check for reentrancy (either by
checking an existing variable or adding a new one to prevent
vu_gpio_stop() from calling itself).

> >   #8  0x0000557adc050999 in qemu_chr_write_buffer (s=0x557adea51f10, 
> > buf=0x7ffe8588cce0 "\v", len=20, offset=0x7ffe8588cbe4, write_all=true) at 
> > ../../chardev/char.c:121
> >   #9  0x0000557adc0507c7 in qemu_chr_write (s=0x557adea51f10, 
> > buf=0x7ffe8588cce0 "\v", len=20, write_all=true) at ../../chardev/char.c:173
> >   #10 0x0000557adc046f3a in qemu_chr_fe_write_all (be=0x557adf80d830, 
> > buf=0x7ffe8588cce0 "\v", len=20) at ../../chardev/char-fe.c:53
> >   #11 0x0000557adbddc02f in vhost_user_write (dev=0x557adf80d878, 
> > msg=0x7ffe8588cce0, fds=0x0, fd_num=0) at ../../hw/virtio/vhost-user.c:490
> >   #12 0x0000557adbddd48f in vhost_user_get_vring_base (dev=0x557adf80d878, 
> > ring=0x7ffe8588d000) at ../../hw/virtio/vhost-user.c:1260
> >   #13 0x0000557adbdd4bd6 in vhost_virtqueue_stop (dev=0x557adf80d878, 
> > vdev=0x557adf80d640, vq=0x557adf843570, idx=0) at 
> > ../../hw/virtio/vhost.c:1220
> >   #14 0x0000557adbdd7eda in vhost_dev_stop (hdev=0x557adf80d878, 
> > vdev=0x557adf80d640, vrings=false) at ../../hw/virtio/vhost.c:1916
> >   #15 0x0000557adbe051a6 in vu_gpio_stop (vdev=0x557adf80d640) at 
> > ../../hw/virtio/vhost-user-gpio.c:142
> >   #16 0x0000557adbe04849 in vu_gpio_set_status (vdev=0x557adf80d640, 
> > status=15 '\017') at ../../hw/virtio/vhost-user-gpio.c:173
> >   #17 0x0000557adbdc87ff in virtio_set_status (vdev=0x557adf80d640, val=15 
> > '\017') at ../../hw/virtio/virtio.c:2442
> >   #18 0x0000557adbdcbfa0 in virtio_vmstate_change (opaque=0x557adf80d640, 
> > running=false, state=RUN_STATE_SHUTDOWN) at ../../hw/virtio/virtio.c:3736
> >   #19 0x0000557adb91ad27 in vm_state_notify (running=false, 
> > state=RUN_STATE_SHUTDOWN) at ../../softmmu/runstate.c:334
> >   #20 0x0000557adb910e88 in do_vm_stop (state=RUN_STATE_SHUTDOWN, 
> > send_stop=false) at ../../softmmu/cpus.c:262
> >   #21 0x0000557adb910e30 in vm_shutdown () at ../../softmmu/cpus.c:280
> >   #22 0x0000557adb91b9c3 in qemu_cleanup () at ../../softmmu/runstate.c:827
> >   #23 0x0000557adb522975 in qemu_default_main () at ../../softmmu/main.c:38
> >   #24 0x0000557adb5229a8 in main (argc=27, argv=0x7ffe8588d2f8) at 
> > ../../softmmu/main.c:48
> >   (rr) p hdev->started
> >   $9 = true
> >   (rr) info thread
> >     Id   Target Id                                Frame
> >   * 1    Thread 2140414.2140414 (qemu-system-aar) vhost_dev_is_started 
> > (hdev=0x557adf80d878) at 
> > /home/alex/lsrc/qemu.git/include/hw/virtio/vhost.h:199
> >     2    Thread 2140414.2140439 (qemu-system-aar) 0x0000000070000002 in 
> > syscall_traced ()
> >     3    Thread 2140414.2140442 (qemu-system-aar) 0x0000000070000002 in 
> > syscall_traced ()
> >     4    Thread 2140414.2140443 (qemu-system-aar) 0x0000000070000002 in 
> > syscall_traced ()
> >
> > During which we eliminate the vhost_dev with a memset:
> >
> >   Thread 1 hit Hardware watchpoint 2: *(unsigned int *) 0x557adf80da30
> >
> >   Old value = 2
> >   New value = 0
> >   __memset_avx2_unaligned_erms () at 
> > ../sysdeps/x86_64/multiarch/memset-vec-unaligned-erms.S:220
> >   Download failed: Invalid argument.  Continuing without source file 
> > ./string/../sysdeps/x86_64/multiarch/memset-vec-unaligned-erms.S.
> >   220     ../sysdeps/x86_64/multiarch/memset-vec-unaligned-erms.S: No such 
> > file or directory.
> >   (rr) bt
> >   #0  __memset_avx2_unaligned_erms () at 
> > ../sysdeps/x86_64/multiarch/memset-vec-unaligned-erms.S:220
> >   #1  0x0000557adbdd67f8 in vhost_dev_cleanup (hdev=0x557adf80d878) at 
> > ../../hw/virtio/vhost.c:1501
> >   #2  0x0000557adbe04d68 in vu_gpio_disconnect (dev=0x557adf80d640) at 
> > ../../hw/virtio/vhost-user-gpio.c:256
> >   #3  0x0000557adbe049bb in vu_gpio_event (opaque=0x557adf80d640, 
> > event=CHR_EVENT_CLOSED) at ../../hw/virtio/vhost-user-gpio.c:274
> >   #4  0x0000557adc0539ef in chr_be_event (s=0x557adea51f10, 
> > event=CHR_EVENT_CLOSED) at ../../chardev/char.c:61
> >   #5  0x0000557adc0506aa in qemu_chr_be_event (s=0x557adea51f10, 
> > event=CHR_EVENT_CLOSED) at ../../chardev/char.c:81
> >   #6  0x0000557adc04f666 in tcp_chr_disconnect_locked (chr=0x557adea51f10) 
> > at ../../chardev/char-socket.c:470
> >   #7  0x0000557adc04c81a in tcp_chr_write (chr=0x557adea51f10, 
> > buf=0x7ffe8588cce0 "\v", len=20) at ../../chardev/char-socket.c:129
> >   #8  0x0000557adc050999 in qemu_chr_write_buffer (s=0x557adea51f10, 
> > buf=0x7ffe8588cce0 "\v", len=20, offset=0x7ffe8588cbe4, write_all=true) at 
> > ../../chardev/char.c:121
> >   #9  0x0000557adc0507c7 in qemu_chr_write (s=0x557adea51f10, 
> > buf=0x7ffe8588cce0 "\v", len=20, write_all=true) at ../../chardev/char.c:173
> >   #10 0x0000557adc046f3a in qemu_chr_fe_write_all (be=0x557adf80d830, 
> > buf=0x7ffe8588cce0 "\v", len=20) at ../../chardev/char-fe.c:53
> >   #11 0x0000557adbddc02f in vhost_user_write (dev=0x557adf80d878, 
> > msg=0x7ffe8588cce0, fds=0x0, fd_num=0) at ../../hw/virtio/vhost-user.c:490
> >   #12 0x0000557adbddd48f in vhost_user_get_vring_base (dev=0x557adf80d878, 
> > ring=0x7ffe8588d000) at ../../hw/virtio/vhost-user.c:1260
> >   #13 0x0000557adbdd4bd6 in vhost_virtqueue_stop (dev=0x557adf80d878, 
> > vdev=0x557adf80d640, vq=0x557adf843570, idx=0) at 
> > ../../hw/virtio/vhost.c:1220
> >   #14 0x0000557adbdd7eda in vhost_dev_stop (hdev=0x557adf80d878, 
> > vdev=0x557adf80d640, vrings=false) at ../../hw/virtio/vhost.c:1916
> >   #15 0x0000557adbe051a6 in vu_gpio_stop (vdev=0x557adf80d640) at 
> > ../../hw/virtio/vhost-user-gpio.c:142
> >   #16 0x0000557adbe04849 in vu_gpio_set_status (vdev=0x557adf80d640, 
> > status=15 '\017') at ../../hw/virtio/vhost-user-gpio.c:173
> >   #17 0x0000557adbdc87ff in virtio_set_status (vdev=0x557adf80d640, val=15 
> > '\017') at ../../hw/virtio/virtio.c:2442
> >   #18 0x0000557adbdcbfa0 in virtio_vmstate_change (opaque=0x557adf80d640, 
> > running=false, state=RUN_STATE_SHUTDOWN) at ../../hw/virtio/virtio.c:3736
> >   #19 0x0000557adb91ad27 in vm_state_notify (running=false, 
> > state=RUN_STATE_SHUTDOWN) at ../../softmmu/runstate.c:334
> >   #20 0x0000557adb910e88 in do_vm_stop (state=RUN_STATE_SHUTDOWN, 
> > send_stop=false) at ../../softmmu/cpus.c:262
> >   #21 0x0000557adb910e30 in vm_shutdown () at ../../softmmu/cpus.c:280
> >   #22 0x0000557adb91b9c3 in qemu_cleanup () at ../../softmmu/runstate.c:827
> >   #23 0x0000557adb522975 in qemu_default_main () at ../../softmmu/main.c:38
> >   #24 0x0000557adb5229a8 in main (argc=27, argv=0x7ffe8588d2f8) at 
> > ../../softmmu/main.c:48
> >
> > Before finally:
> >
> >   #0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
> >   #1  0x00007f24dc269537 in __GI_abort () at abort.c:79
> >   #2  0x00007f24dc26940f in __assert_fail_base (fmt=0x7f24dc3e16a8 
> > "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", assertion=0x557adc28d8f5 
> > "assign || nvqs == proxy->nvqs_with_notifiers", file=0x557adc28d7ab 
> > "../../hw/virtio/virtio-pci.c", line=1029, function=<optimized out>) at 
> > assert.c:92
> >   #3  0x00007f24dc278662 in __GI___assert_fail (assertion=0x557adc28d8f5 
> > "assign || nvqs == proxy->nvqs_with_notifiers", file=0x557adc28d7ab 
> > "../../hw/virtio/virtio-pci.c", line=1029, function=0x557adc28d922 "int 
> > virtio_pci_set_guest_notifiers(DeviceState *, int, _Bool)") at assert.c:101
> >   #4  0x0000557adb8e97f1 in virtio_pci_set_guest_notifiers 
> > (d=0x557adf805280, nvqs=0, assign=false) at 
> > ../../hw/virtio/virtio-pci.c:1029
> >   #5  0x0000557adbe051c7 in vu_gpio_stop (vdev=0x557adf80d640) at 
> > ../../hw/virtio/vhost-user-gpio.c:144
> >   #6  0x0000557adbe04849 in vu_gpio_set_status (vdev=0x557adf80d640, 
> > status=15 '\017') at ../../hw/virtio/vhost-user-gpio.c:173
> >   #7  0x0000557adbdc87ff in virtio_set_status (vdev=0x557adf80d640, val=15 
> > '\017') at ../../hw/virtio/virtio.c:2442
> >   #8  0x0000557adbdcbfa0 in virtio_vmstate_change (opaque=0x557adf80d640, 
> > running=false, state=RUN_STATE_SHUTDOWN) at ../../hw/virtio/virtio.c:3736
> >   #9  0x0000557adb91ad27 in vm_state_notify (running=false, 
> > state=RUN_STATE_SHUTDOWN) at ../../softmmu/runstate.c:334
> >   #10 0x0000557adb910e88 in do_vm_stop (state=RUN_STATE_SHUTDOWN, 
> > send_stop=false) at ../../softmmu/cpus.c:262
> >   #11 0x0000557adb910e30 in vm_shutdown () at ../../softmmu/cpus.c:280
> >   #12 0x0000557adb91b9c3 in qemu_cleanup () at ../../softmmu/runstate.c:827
> >   #13 0x0000557adb522975 in qemu_default_main () at ../../softmmu/main.c:38
> >   #14 0x0000557adb5229a8 in main (argc=27, argv=0x7ffe8588d2f8) at 
> > ../../softmmu/main.c:48
> >
> > Because of course we've just done that on disconnect.
> >
> > Not sure what the cleanest way to avoid that is yet. Maybe it will be
> > clearer on Monday morning.
>
>
> --
> Alex Bennée
>