Re: [PATCH v2 05/11] locking/ww_mutex: Add waiters in stamp order

[Chris Wilson]

On Thu, Dec 01, 2016 at 03:06:48PM +0100, Nicolai Hähnle wrote:
> @@ -677,15 +722,25 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  	debug_mutex_lock_common(lock, &waiter);
>  	debug_mutex_add_waiter(lock, &waiter, task);
>  
> -	/* add waiting tasks to the end of the waitqueue (FIFO): */
> -	list_add_tail(&waiter.list, &lock->wait_list);
> +	lock_contended(&lock->dep_map, ip);
> +
> +	if (!use_ww_ctx) {
> +		/* add waiting tasks to the end of the waitqueue (FIFO): */
> +		list_add_tail(&waiter.list, &lock->wait_list);
> +	} else {
> +		/* Add in stamp order, waking up waiters that must back off. */
> +		ret = __ww_mutex_add_waiter(&waiter, lock, ww_ctx);
> +		if (ret)
> +			goto err_early_backoff;
> +
> +		waiter.ww_ctx = ww_ctx;
> +	}
> +
>  	waiter.task = task;

Would an unconditional waiter.ww_ctx = ww_ctx be chep enough? (Same
cacheline write and all that?)

Makes the above clearer in that you have

	if (!ww_ctx) {
		list_add_tail();
	} else {
		ret = __ww_mutex_add_waiter(); /* no need to handle !ww_ctx */
		if (ret)
			goto err_early_backoff;
	}

	waiter.ww_ctx = ww_ctx;
	waiter.task = task;

>  
>  	if (__mutex_waiter_is_first(lock, &waiter))
>  		__mutex_set_flag(lock, MUTEX_FLAG_WAITERS);
>  
> -	lock_contended(&lock->dep_map, ip);
> -
>  	set_task_state(task, state);
>  	for (;;) {
>  		/*
> @@ -693,8 +748,12 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  		 * mutex_unlock() handing the lock off to us, do a trylock
>  		 * before testing the error conditions to make sure we pick up
>  		 * the handoff.
> +		 *
> +		 * For w/w locks, we always need to do this even if we're not
> +		 * currently the first waiter, because we may have been the
> +		 * first waiter during the unlock.
>  		 */
> -		if (__mutex_trylock(lock, first))
> +		if (__mutex_trylock(lock, use_ww_ctx || first))

I'm not certain about the magic of first vs HANDOFF. Afaict, first ==
HANDOFF and this patch breaks that relationship. I think you need to add
bool handoff; as a separate tracker to first.

>  			goto acquired;
>  
>  		/*
> @@ -716,7 +775,20 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  		spin_unlock_mutex(&lock->wait_lock, flags);
>  		schedule_preempt_disabled();
>  
> -		if (!first && __mutex_waiter_is_first(lock, &waiter)) {
> +		if (use_ww_ctx && ww_ctx) {
> +			/*
> +			 * Always re-check whether we're in first position. We
> +			 * don't want to spin if another task with a lower
> +			 * stamp has taken our position.
> +			 *
> +			 * We also may have to set the handoff flag again, if
> +			 * our position at the head was temporarily taken away.

Comment makes sense.

Ah. Should this be just if (use_ww_ctx) { /* always recheck... */ ?
Except that !ww_ctx are never gazzumped in the list, so if they are
first, then they are always first.

Could you explain that as well (about why !ww_ctx is special here but
not above). And then it can even be reduced to if (ww_ctx) {} to match
the first chunk if the revision is acceptable.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre