[PATCH v3 2/2] locking: Implement an algorithm choice for Wound-Wait mutexes

[Peter Zijlstra]

On Fri, Jun 15, 2018 at 02:08:27PM +0200, Thomas Hellstrom wrote:

> @@ -772,6 +856,25 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
>  	}
>  
>  	list_add_tail(&waiter->list, pos);
> +	if (__mutex_waiter_is_first(lock, waiter))
> +		__mutex_set_flag(lock, MUTEX_FLAG_WAITERS);
> +
> +	/*
> +	 * Wound-Wait: if we're blocking on a mutex owned by a younger context,
> +	 * wound that such that we might proceed.
> +	 */
> +	if (!is_wait_die) {
> +		struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
> +
> +		/*
> +		 * See ww_mutex_set_context_fastpath(). Orders setting
> +		 * MUTEX_FLAG_WAITERS (atomic operation) vs the ww->ctx load,
> +		 * such that either we or the fastpath will wound @ww->ctx.
> +		 */
> +		smp_mb__after_atomic();
> +
> +		__ww_mutex_wound(lock, ww_ctx, ww->ctx);
> +	}

I think we want the smp_mb__after_atomic() in the same branch as
__mutex_set_flag(). So something like:

	if (__mutex_waiter_is_first()) {
		__mutex_set_flag();
		if (!is_wait_die)
			smp_mb__after_atomic();
	}

Or possibly even without the !is_wait_die. The rules for
smp_mb__*_atomic() are such that we want it unconditional after an
atomic, otherwise the semantics get too fuzzy.

Alan (rightfully) complained about that a while ago when he was auditing
users.