Commit 69de4844 authored Oct 03, 2019 by Stefan Hajnoczi

test-bdrv-drain: fix iothread_join() hang



tests/test-bdrv-drain can hang in tests/iothread.c:iothread_run():

  while (!atomic_read(&iothread->stopping)) {
      aio_poll(iothread->ctx, true);
  }

The iothread_join() function works as follows:

  void iothread_join(IOThread *iothread)
  {
      iothread->stopping = true;
      aio_notify(iothread->ctx);
      qemu_thread_join(&iothread->thread);

If iothread_run() checks iothread->stopping before the iothread_join()
thread sets stopping to true, then aio_notify() may be optimized away
and iothread_run() hangs forever in aio_poll().

The correct way to change iothread->stopping is from a BH that executes
within iothread_run().  This ensures that iothread->stopping is checked
after we set it to true.

This was already fixed for ./iothread.c (note this is a different source
file!) by commit 2362a28e ("iothread:
fix iothread_stop() race condition"), but not for tests/iothread.c.

Fixes: 0c330a73
       ("aio: introduce aio_co_schedule and aio_co_wake")
Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20191003100103.331-1-stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

parent 98b2e3c9

tests/iothread.c

+8 −2

Original line number	Diff line number	Diff line
		@@ -55,10 +55,16 @@ static void iothread_run(void opaque)
		return NULL;
		}

		void iothread_join(IOThread *iothread)
		static void iothread_stop_bh(void *opaque)
		{
		IOThread *iothread = opaque;

		iothread->stopping = true;
		aio_notify(iothread->ctx);
		}

		void iothread_join(IOThread *iothread)
		{
		aio_bh_schedule_oneshot(iothread->ctx, iothread_stop_bh, iothread);
		qemu_thread_join(&iothread->thread);
		qemu_cond_destroy(&iothread->init_done_cond);
		qemu_mutex_destroy(&iothread->init_done_lock);