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

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Date:   Tue, 6 Dec 2016 16:36:06 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nicolai Hähnle <nhaehnle@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Nicolai Hähnle <Nicolai.Haehnle@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Maarten Lankhorst <dev@...ankhorst.nl>,
        Daniel Vetter <daniel@...ll.ch>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v2 05/11] locking/ww_mutex: Add waiters in stamp order

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.