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[Qemu-block] [PULL 1/1] virtio-blk: Cancel the pending BH when the datap

From: Stefan Hajnoczi
Subject: [Qemu-block] [PULL 1/1] virtio-blk: Cancel the pending BH when the dataplane is reset
Date: Wed, 11 Sep 2019 16:36:12 +0200

From: Philippe Mathieu-Daudé <address@hidden>

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

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 \
     -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.

  (gdb) bt
  Thread 1 (Thread 0x7f109c17b680 (LWP 10939)):
  #0  0x00005604083296d1 in vring_get_region_caches (vq=0x56040a24bdd0) at 
  #1  0x000056040832972b in vring_avail_flags (vq=0x56040a24bdd0) at 
  #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 
  #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 
  #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 
  #10 0x0000560408841445 in glib_pollfds_poll () at util/main-loop.c:215
  #11 0x00005604088414bf in os_host_main_loop_wait (timeout=0) at 
  #12 0x00005604088415c4 in main_loop_wait (nonblocking=0) at 
  #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.

[This is version of the patch was modified as discussed with Philippe on
the mailing list thread.

Reported-by: Yihuang Yu <address@hidden>
Suggested-by: Stefan Hajnoczi <address@hidden>
Fixes: https://bugs.launchpad.net/qemu/+bug/1839428
Signed-off-by: Philippe Mathieu-Daudé <address@hidden>
Message-Id: <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>
 hw/block/dataplane/virtio-blk.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index 9299a1a7c2..119906a5fe 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -298,6 +298,9 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
         virtio_bus_cleanup_host_notifier(VIRTIO_BUS(qbus), i);
+    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);

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