linux-kernel - Re: [PATCH v3 2/3] mutex: add support for wound/wait style locks, v3

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Date:	Mon, 27 May 2013 10:21:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	x86@...nel.org, dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org, robclark@...il.com,
	rostedt@...dmis.org, tglx@...utronix.de, mingo@...e.hu,
	linux-media@...r.kernel.org, Dave Airlie <airlied@...hat.com>
Subject: Re: [PATCH v3 2/3] mutex: add support for wound/wait style locks, v3

On Wed, May 22, 2013 at 07:24:38PM +0200, Maarten Lankhorst wrote:
> >> +static inline void ww_acquire_init(struct ww_acquire_ctx *ctx,
> >> +				   struct ww_class *ww_class)
> >> +{
> >> +	ctx->task = current;
> >> +	do {
> >> +		ctx->stamp = atomic_long_inc_return(&ww_class->stamp);
> >> +	} while (unlikely(!ctx->stamp));
> > I suppose we'll figure something out when this becomes a bottleneck. Ideally
> > we'd do something like:
> >
> >  ctx->stamp = local_clock();
> >
> > but for now we cannot guarantee that's not jiffies, and I suppose that's a tad
> > too coarse to work for this.
> This might mess up when 2 cores happen to return exactly the same time, how do you choose a winner in that case?
> EDIT: Using pointer address like you suggested below is fine with me. ctx pointer would be static enough.

Right, but for now I suppose the 'global' atomic is ok, if/when we find
it hurts performance we can revisit. I was just spewing ideas :-)

> > Also, why is 0 special?
> Oops, 0 is no longer special.
> 
> I used to set the samp directly on the lock, so 0 used to mean no ctx set.

Ah, ok :-)

> >> +static inline int __must_check ww_mutex_trylock_single(struct ww_mutex *lock)
> >> +{
> >> +	return mutex_trylock(&lock->base);
> >> +}
> > trylocks can never deadlock they don't block per definition, I don't see the
> > point of the _single() thing here.
> I called it single because they weren't annotated into any ctx. I can drop the _single suffix though,
> but you'd still need to unlock with unlock_single, or we need to remove that distinction altogether,
> lose a few lockdep checks and only have a one unlock function.

Again, early.. monday.. would a trylock, even if successful still need
the ctx?


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