Re: [RFC PATCH 4/4] coroutine/rwlock: Wake writers in preference to readers

[Paolo Bonzini <pbonzini@redhat.com>]

On 09/03/21 11:21, David Edmondson wrote:
> -        /* The critical section started in qemu_co_rwlock_wrlock.  */
> -        qemu_co_queue_restart_all(&lock->queue);
> +        /* The critical section started in qemu_co_rwlock_wrlock or
> +         * qemu_co_rwlock_upgrade.
> +         */
> +        qemu_co_queue_restart_all(&lock->wqueue);
> +        qemu_co_queue_restart_all(&lock->rqueue);

Hmm, the devil is in the details---this is a thundering herd waiting to 
happen.  But fortunately this can be fixed while making the unlock 
primitive even simpler:

     if (lock->reader) {
          self->locks_held--;

          /* Read-side critical sections do not keep lock->mutex.  */
          qemu_co_mutex_lock(&lock->mutex);
          lock->reader--;
          assert(lock->reader >= 0);
      }

      /* If there are no remaining readers wake one waiting writer
       * or all waiting readers.
       */
      if (!lock->reader && !qemu_co_queue_next(&lock->wqueue)) {
          assert(!lock->pending_writer);
          qemu_co_queue_restart_all(&lock->rqueue);
      }

Thanks,

Paolo