Re: aio_wait_bh_oneshot() thread-safety question

[Paolo Bonzini <pbonzini@redhat.com>]

On 5/24/22 14:40, Kevin Wolf wrote:
> Why is the barrier in aio_bh_enqueue() not enough? Is the comment there
> wrong?
> 
> aio_notify() has another barrier. This is a little bit too late, but if
> I misunderstood the aio_bh_enqueue() one, it could explain why it was
> never observed.

The missing one that I (and I think Vladimir) were talking about is at the
end of the execution of the bottom half, not at the beginning:

/* Context: BH in IOThread */
static void aio_wait_bh(void *opaque)
{
     AioWaitBHData *data = opaque;

     data->cb(data->opaque);

     data->done = true;
     aio_wait_kick();
}

void aio_wait_kick(void)
{
     /* The barrier (or an atomic op) is in the caller.  */
     if (qatomic_read(&global_aio_wait.num_waiters)) {
         aio_bh_schedule_oneshot(qemu_get_aio_context(), dummy_bh_cb, NULL);
     }
}

where there is no barrier in the caller to separate reading data->done
(qatomic_set would be nice, if only for clarity) from reading num_waiters.

Paolo