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.