Re: [PATCH v2 3/3] aio-posix: keep aio_notify_me disabled during polling

[Paolo Bonzini <pbonzini@redhat.com>]

On 05/08/20 12:00, Stefan Hajnoczi wrote:
> +
> +        /*
> +         * aio_notify can avoid the expensive event_notifier_set if
> +         * everything (file descriptors, bottom halves, timers) will
> +         * be re-evaluated before the next blocking poll().  This is
> +         * already true when aio_poll is called with blocking == false;
> +         * if blocking == true, it is only true after poll() returns,
> +         * so disable the optimization now.
> +         */
> +        if (use_notify_me) {
> +            atomic_set(&ctx->notify_me, atomic_read(&ctx->notify_me) + 2);
> +            /*
> +             * Write ctx->notify_me before reading ctx->notified.  Pairs with
> +             * smp_mb in aio_notify().
> +             */
> +            smp_mb();
> +
> +            /* Don't block if aio_notify() was called */
> +            if (atomic_read(&ctx->notified)) {
> +                timeout = 0;
> +            }

Aha, this is the trick: "timeout = 0" also applies if a timer was moved 
early.  In this case you uselessly keep notify_me set for a bit, but 
it's okay. Nice!

The code can be simplified a bit more, since the use_notify_me variable 
is just "timeout":

    use_notify_me = (timeout != 0);
    if (use_notify_me) {
         /*
          * aio_notify can avoid the expensive event_notifier_set if
          * everything (file descriptors, bottom halves, timers) will
          * be re-evaluated before the next blocking poll().  This is
          * already true when aio_poll is called with blocking == false;
          * if blocking == true, it is only true after poll() returns,
          * so disable the optimization now.
          */
         atomic_set(&ctx->notify_me, atomic_read(&ctx->notify_me) + 2);
         /*
          * Write ctx->notify_me before reading ctx->notified.  Pairs with
          * smp_mb in aio_notify().
          */
         smp_mb();
 
         /* Don't block if aio_notify() was called */
         if (atomic_read(&ctx->notified)) {
             timeout = 0;
         }
     }
     if (timeout || ctx->fdmon_ops->need_wait(ctx)) {
         ret = ctx->fdmon_ops->wait(ctx, &ready_list, timeout);
     }
     if (use_notify_me) {
         /* Finish the poll before clearing the flag.  */
         atomic_store_release(&ctx->notify_me,
                              atomic_read(&ctx->notify_me) - 2);
     }

Paolo