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[Qemu-devel] [PATCH v2 1/1] virtio-blk: fix race on guest notifiers

From: Halil Pasic
Subject: [Qemu-devel] [PATCH v2 1/1] virtio-blk: fix race on guest notifiers
Date: Tue, 7 Mar 2017 14:16:50 +0100

The commits 03de2f527 "virtio-blk: do not use vring in dataplane"  and
9ffe337c08 "virtio-blk: always use dataplane path if ioeventfd is active"
changed how notifications are done for virtio-blk substantially. Due to a
race condition, interrupts are lost when irqfd behind the guest notifier
is torn down after notify_guest_bh was scheduled but before it actually

Let's fix this by forcing guest notifications before cleaning up the
irqfd's. Let's also add some explanatory comments.

Cc: address@hidden
Signed-off-by: Halil Pasic <address@hidden>
Reported-by: Michael A. Tebolt <address@hidden>
Tested-by: Michael A. Tebolt <address@hidden>
Suggested-by: Paolo Bonzini <address@hidden>

This patch withstood the test case which discovered the problem
for several days (as reported by Michale Tebolt).

v1 --> v2:
* Fixed typo pointed out by Connie
* Added Tested-by

 hw/block/dataplane/virtio-blk.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index 5556f0e..045a580 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -258,9 +258,16 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
         virtio_queue_aio_set_host_notifier_handler(vq, s->ctx, NULL);
-    /* Drain and switch bs back to the QEMU main loop */
+    /* Drain and switch bs back to the QEMU main loop. After drain, the
+     * device will not submit (nor complete) any requests until dataplane
+     * starts again.
+     */
     blk_set_aio_context(s->conf->conf.blk, qemu_get_aio_context());
+    /* Notify guest before the guest notifiers get cleaned up */
+    qemu_bh_cancel(s->bh);
+    notify_guest_bh(s);
     for (i = 0; i < nvqs; i++) {

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