Commit dce8921b2baaf95974af8176406881872067adfa ("iothread: Stop threads
before main() quits") introduced iothread_stop_all() to avoid the
following virtio-scsi assertion failure:
assert(blk_get_aio_context(d->conf.blk) == s->ctx);
Back then the assertion failed because when bdrv_close_all() made
d->conf.blk NULL, blk_get_aio_context() returned the global AioContext
instead of s->ctx.
The same assertion can still fail today when vcpus submit new I/O
requests after iothread_stop_all() has moved the BDS to the global
AioContext.
This patch hardens the iothread_stop_all() approach by pausing vcpus
before calling iothread_stop_all().
Note that the assertion failure is a race condition. It is not possible
to reproduce it reliably.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
v2:
* Add comment explaining rationale for this ordering and why it's safe.
[Kevin]
vl.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)
diff --git a/vl.c b/vl.c
index e517a8d995..b72a9fc6df 100644
--- a/vl.c
+++ b/vl.c
@@ -4766,10 +4766,18 @@ int main(int argc, char **argv, char **envp)
main_loop();
replay_disable_events();
+
+ /* The ordering of the following is delicate. Stop vcpus to prevent new
+ * I/O requests being queued by the guest. Then stop IOThreads (this
+ * includes a drain operation and completes all request processing). At
+ * this point emulated devices are still associated with their IOThreads
+ * (if any) but no longer have any work to do. Only then can we close
+ * block devices safely because we know there is no more I/O coming.
+ */
+ pause_all_vcpus();
iothread_stop_all();
-
- pause_all_vcpus();
bdrv_close_all();
+
res_free();
/* vhost-user must be cleaned up before chardevs. */
--
2.14.3