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vl: pause vcpus before stopping iothreads

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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>
Message-id: 20180201110708.8080-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2018-02-01 11:07:08 +00:00
parent bec9c64ef7
commit 00d09fdbba
1 changed files with 9 additions and 1 deletions
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10
vl.c
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@ -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. */