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

[Nicolai HÃhnle]

On 06.12.2016 16:36, Peter Zijlstra wrote:
> On Thu, Dec 01, 2016 at 03:06:48PM +0100, Nicolai Hähnle wrote:
> > +static inline int __sched
> > +__ww_mutex_add_waiter(struct mutex_waiter *waiter,
> > +		      struct mutex *lock,
> > +		      struct ww_acquire_ctx *ww_ctx)
> > +{
> > +	struct mutex_waiter *cur;
> > +
> > +	if (!ww_ctx) {
> > +		list_add_tail(&waiter->list, &lock->wait_list);
> > +		return 0;
> > +	}
> > +
> > +	/*
> > +	 * Add the waiter before the first waiter with a higher stamp.
> > +	 * Waiters without a context are skipped to avoid starving
> > +	 * them.
> > +	 */
> > +	list_for_each_entry(cur, &lock->wait_list, list) {
> > +		if (!cur->ww_ctx)
> > +			continue;
> > +
> > +		if (__ww_mutex_stamp_after(ww_ctx, cur->ww_ctx)) {
> > +			/* Back off immediately if necessary. */
> > +			if (ww_ctx->acquired > 0) {
> > +#ifdef CONFIG_DEBUG_MUTEXES
> > +				struct ww_mutex *ww;
> > +
> > +				ww = container_of(lock, struct ww_mutex, base);
> > +				DEBUG_LOCKS_WARN_ON(ww_ctx->contending_lock);
> > +				ww_ctx->contending_lock = ww;
> > +#endif
> > +				return -EDEADLK;
> > +			}
> > +
> > +			continue;
> > +		}
> > +
> > +		list_add_tail(&waiter->list, &cur->list);
> > +		return 0;
> > +	}
> > +
> > +	list_add_tail(&waiter->list, &lock->wait_list);
> > +	return 0;
> > +}
>
> So you keep the list in order of stamp, and in general stamps come in,
> in-order. That is, barring races on concurrent ww_mutex_lock(), things
> are already ordered.
>
> So it doesn't make sense to scan the entire list forwards, that's almost
> guarantees you scan the entire list every single time.
>
> Or am I reading this wrong? Which in itself is a hint a comment might be
> in place.

No, it's a reasonable question. Some things to keep in mind:

1. Each ww_acquire_ctx may be used with hundreds of locks. It's not that 
clear that things will be ordered in a contention scenario, especially 
since the old stamp is re-used when a context backs off and goes into 
the slow path (with ww_ctx->acquired == 0).

2. We want to add waiters directly before the first waiter with a higher 
stamp. Keeping in mind that there may be non-ww_ctx waiters in between, 
and we don't want to starve them, traversing the list backwards would 
require keeping the best insertion point around in a separate variable. 
Clearly possible, but it seemed more awkward.

In hindsight, backwards iteration may not be so bad.

Nicolai