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.
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