From 00d09fdbbae5f7864ce754913efc84c12fdf9f1a Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Thu, 1 Feb 2018 11:07:08 +0000 Subject: [PATCH] vl: pause vcpus before stopping iothreads 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 Message-id: 20180201110708.8080-1-stefanha@redhat.com Signed-off-by: Stefan Hajnoczi --- vl.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/vl.c b/vl.c index 21878496ec..7a5554bc41 100644 --- a/vl.c +++ b/vl.c @@ -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. */