Re: [Qemu-devel] [PATCH] virtio-blk: Cancel the pending BH when the dataplane is reset

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 8/22/19 3:01 PM, Stefan Hajnoczi wrote:
> On Fri, Aug 16, 2019 at 07:15:03PM +0200, Philippe Mathieu-Daudé wrote:
>> When 'system_reset' is called, the main loop clear the memory
>> region cache before the BH has a chance to execute. Later when
>> the deferred function is called, some assumptions that were
>> made when scheduling them are no longer true when they actually
>> execute.
>>
>> This is what happens using a virtio-blk device (fresh RHEL7.8 install):
>>
>>  $ (sleep 12.3; echo system_reset; sleep 12.3; echo system_reset; sleep 1; echo q) \
>>    | qemu-system-x86_64 -m 4G -smp 8 -boot menu=on \
>>      -device virtio-blk-pci,id=image1,drive=drive_image1 \
>>      -drive file=/var/lib/libvirt/images/rhel78.qcow2,if=none,id=drive_image1,format=qcow2,cache=none \
>>      -device virtio-net-pci,netdev=net0,id=nic0,mac=52:54:00:c4:e7:84 \
>>      -netdev tap,id=net0,script=/bin/true,downscript=/bin/true,vhost=on \
>>      -monitor stdio -serial null -nographic
>>   (qemu) system_reset
>>   (qemu) system_reset
>>   (qemu) qemu-system-x86_64: hw/virtio/virtio.c:225: vring_get_region_caches: Assertion `caches != NULL' failed.
>>   Aborted
>>
>>   (gdb) bt
>>   Thread 1 (Thread 0x7f109c17b680 (LWP 10939)):
>>   #0  0x00005604083296d1 in vring_get_region_caches (vq=0x56040a24bdd0) at hw/virtio/virtio.c:227
>>   #1  0x000056040832972b in vring_avail_flags (vq=0x56040a24bdd0) at hw/virtio/virtio.c:235
>>   #2  0x000056040832d13d in virtio_should_notify (vdev=0x56040a240630, vq=0x56040a24bdd0) at hw/virtio/virtio.c:1648
>>   #3  0x000056040832d1f8 in virtio_notify_irqfd (vdev=0x56040a240630, vq=0x56040a24bdd0) at hw/virtio/virtio.c:1662
>>   #4  0x00005604082d213d in notify_guest_bh (opaque=0x56040a243ec0) at hw/block/dataplane/virtio-blk.c:75
>>   #5  0x000056040883dc35 in aio_bh_call (bh=0x56040a243f10) at util/async.c:90
>>   #6  0x000056040883dccd in aio_bh_poll (ctx=0x560409161980) at util/async.c:118
>>   #7  0x0000560408842af7 in aio_dispatch (ctx=0x560409161980) at util/aio-posix.c:460
>>   #8  0x000056040883e068 in aio_ctx_dispatch (source=0x560409161980, callback=0x0, user_data=0x0) at util/async.c:261
>>   #9  0x00007f10a8fca06d in g_main_context_dispatch () at /lib64/libglib-2.0.so.0
>>   #10 0x0000560408841445 in glib_pollfds_poll () at util/main-loop.c:215
>>   #11 0x00005604088414bf in os_host_main_loop_wait (timeout=0) at util/main-loop.c:238
>>   #12 0x00005604088415c4 in main_loop_wait (nonblocking=0) at util/main-loop.c:514
>>   #13 0x0000560408416b1e in main_loop () at vl.c:1923
>>   #14 0x000056040841e0e8 in main (argc=20, argv=0x7ffc2c3f9c58, envp=0x7ffc2c3f9d00) at vl.c:4578
>>
>> Fix this by cancelling the BH when the virtio dataplane is stopped.
>>
>> Reported-by: Yihuang Yu <yihyu@redhat.com>
>> Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
>> Fixes: https://bugs.launchpad.net/qemu/+bug/1839428
>> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>> ---
>>  hw/block/dataplane/virtio-blk.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
>> index 9299a1a7c2..4030faa21d 100644
>> --- a/hw/block/dataplane/virtio-blk.c
>> +++ b/hw/block/dataplane/virtio-blk.c
>> @@ -301,6 +301,8 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
>>      /* Clean up guest notifier (irq) */
>>      k->set_guest_notifiers(qbus->parent, nvqs, false);
>>  
>> +    qemu_bh_cancel(s->bh);
>> +
>>      vblk->dataplane_started = false;
>>      s->stopping = false;
>>  }
>> -- 
>> 2.20.1
>>
> 
> Along the lines of what John said:
> 
> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
> index 9299a1a7c2..4030faa21d 100644
> --- a/hw/block/dataplane/virtio-blk.c
> +++ b/hw/block/dataplane/virtio-blk.c
> @@ -301,6 +301,8 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
> +    qemu_bh_cancel(s->bh);
> +    notify_guest_bh(s); /* final chance to notify guest */
> +
>      /* Clean up guest notifier (irq) */
>      k->set_guest_notifiers(qbus->parent, nvqs, false);
>  
>      vblk->dataplane_started = false;
>      s->stopping = false;
>  }
> 
> Let's notify the guest if any batched notifications are waiting.  This
> ensures that no notifications are lost when dataplane is stopped.

OK, works for me, thanks!