Commit 00d09fdb authored Feb 01, 2018 by Stefan Hajnoczi

vl: pause vcpus before stopping iothreads



Commit dce8921b ("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>
Message-id: 20180201110708.8080-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

parent bec9c64e

vl.c

+9 −1

Original line number	Diff line number	Diff line
		@@ -4767,10 +4767,18 @@ int main(int argc, char argv, char envp)

		main_loop();
		replay_disable_events();
		iothread_stop_all();

		/* 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();
		bdrv_close_all();

		res_free();

		/* vhost-user must be cleaned up before chardevs. */