The tests/test-bdrv-drain /bdrv-drain/iothread/drain test case does the
following:

1. The preadv coroutine calls aio_bh_schedule_oneshot() and then yields.
2. The one-shot BH executes in another AioContext.  All it does is call
   aio_co_wakeup(preadv_co).
3. The preadv coroutine is re-entered and returns.

There is a race condition in aio_co_wake() where the preadv coroutine
returns and the test case destroys the preadv IOThread.  aio_co_wake()
can still be running in the other AioContext and it performs an access
to the freed IOThread AioContext.

Here is the race in aio_co_schedule():

  QSLIST_INSERT_HEAD_ATOMIC(&ctx->scheduled_coroutines,
                            co, co_scheduled_next);
  <-- race: co may execute before we invoke qemu_bh_schedule()!
  qemu_bh_schedule(ctx->co_schedule_bh);

So if co causes ctx to be freed then we're in trouble.  Fix this problem
by holding a reference to ctx.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20190723190623.21537-1-stefanha@redhat.com
Message-Id: <20190723190623.21537-1-stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>