On 12/02, Davidlohr Bueso wrote:
>
> @@ -102,8 +103,13 @@ void __percpu_up_read(struct percpu_rw_semaphore *sem)
> */
> __this_cpu_dec(*sem->read_count);
>
> + rcu_read_lock();
> + writer = rcu_dereference(sem->writer);
> +
> /* Prod writer to recheck readers_active */
> - wake_up(&sem->writer);
> + if (writer)
> + wake_up_process(writer);
> + rcu_read_unlock();
This needs a barrier between __this_cpu_dec() and rcu_dereference(), I think.
> @@ -159,8 +165,18 @@ void percpu_down_write(struct percpu_rw_semaphore *sem)
> * will wait for them.
> */
>
> - /* Wait for all now active readers to complete. */
> - wait_event(sem->writer, readers_active_check(sem));
> + WRITE_ONCE(sem->writer, current);
> + for (;;) {
> + set_current_state(TASK_UNINTERRUPTIBLE);
> +
> + if (readers_active_check(sem))
> + break;
This looks fine, we can rely on set_current_state() which inserts a barrier
between WRITE_ONCE() and readers_active_check(). So we do not even need
WRITE_ONCE().
And the fact this needs the barriers and the comments makes me think again
you should add the new helpers.
Oleg.
