Re: [Qemu-block] [PATCH for-2.11 3/4] coroutine: Cancel aio_co_schedule() on direct entry

[Kevin Wolf]

Am 28.11.2017 um 17:14 hat Paolo Bonzini geschrieben:
> On 28/11/2017 16:43, Kevin Wolf wrote:
> > +    /* Make sure that a coroutine that can alternatively reentered from two
> > +     * different sources isn't reentered more than once when the first 
> > caller
> > +     * uses aio_co_schedule() and the other one enters to coroutine 
> > directly.
> > +     * This is achieved by cancelling the pending aio_co_schedule().
> > +     *
> > +     * The other way round, if aio_co_schedule() would be called after this
> > +     * point, this would be a problem, too, but in practice it doesn't 
> > happen
> > +     * because we're holding the AioContext lock here and aio_co_schedule()
> > +     * callers must do the same.
> 
> No, this is not true.  aio_co_schedule is thread-safe.

Hm... With the reproducer we were specfically looking at
qmp_block_job_cancel(), which does take the AioContext locks. But it
might not be as universal as I thought.

To be honest, I just wasn't sure what to do with this case anyway. It
means that the coroutine is already running when someone else schedules
it. We don't really know whether we have to enter it a second time or
not.

So if it can indeed happen in practice, we need to think a bit more
about this.

> > This means that the coroutine just needs to
> > +     * prevent other callers from calling aio_co_schedule() before it 
> > yields
> > +     * (e.g. block job coroutines by setting job->busy = true).
> > +     *
> > +     * We still want to ensure that the second case doesn't happen, so 
> > reset
> > +     * co->scheduled only after setting co->caller to make the above check
> > +     * effective for the co_schedule_bh_cb() case. */
> > +    atomic_set(&co->scheduled, NULL);
> 
> This doesn't work.  The coroutine is still in the list, and if someone
> calls aio_co_schedule again now, any coroutines linked from "co" via
> co_scheduled_next are lost.

Why would they? We still iterate the whole list in co_schedule_bh_cb(),
we just skip the single qemu_coroutine_enter().

> (Also, the AioContext lock is by design not protecting any state in
> AioContext itself; the AioContext lock is only protecting things that
> run in an AioContext but do not have their own lock).

Such as the coroutine we want to enter, no?

Kevin