linux-kernel - Re: [PATCH 4/4] locking: Add kselftests for ww

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Date:   Wed, 30 Nov 2016 12:52:29 +0000
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Maarten Lankhorst <dev@...ankhorst.nl>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Nicolai Hähnle <nhaehnle@...il.com>
Subject: Re: [PATCH 4/4] locking: Add kselftests for ww_mutex stress

On Wed, Nov 30, 2016 at 01:29:39PM +0100, Maarten Lankhorst wrote:
> > +static void stress_work(struct work_struct *work)
> > +{
> > +	struct stress *stress = container_of(work, typeof(*stress), work);
> > +	const int nlocks = stress->nlocks;
> > +	struct ww_mutex *locks = stress->locks;
> > +	struct ww_acquire_ctx ctx;
> > +	int contended = -1;
> > +	int *order;
> > +	int n, ret;
> > +
> > +	order = get_random_order(nlocks);
> > +	if (!order)
> > +		return;
> > +
> > +	ww_acquire_init(&ctx, &ww_class);
> > +
> > +retry:
> > +	ret = 0;
> > +	for (n = 0; n < nlocks; n++) {
> > +		if (n == contended)
> > +			continue;
> > +
> > +		ret = ww_mutex_lock(&locks[order[n]], &ctx);
> > +		if (ret < 0)
> > +			break;
> > +	}
> What's wrong with attempting to lock the contended lock here?
> Who knows, this might find some more bugs than the functional tests already do.

I was trying to follow the guide, which was lock, backoff by unlocking
everything, slowlock the contended lock, then lock everything else.

I have now a second worker that follows the reordering method as well.
(As well as a test that slowlock after the ABBA deadlock detection
resolves the locking order.)

If you have a sketch of something else to try, I'll add it.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre