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 12:24:57 +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 Mon, May 27, 2013 at 12:01:50PM +0200, Maarten Lankhorst wrote:
> > Again, early.. monday.. would a trylock, even if successful still need
> > the ctx?
> No ctx for trylock is supported. You can still do a trylock while
> holding a context, but the mutex won't be a part of the context.
> Normal lockdep rules apply. lib/locking-selftest.c:
> 
> context + ww_mutex_lock first, then a trylock:
> dotest(ww_test_context_try, SUCCESS, LOCKTYPE_WW);
> 
> trylock first, then context + ww_mutex_lock:
> dotest(ww_test_try_context, FAILURE, LOCKTYPE_WW);
> 
> For now I don't want to add support for a trylock with context, I'm
> very glad I managed to fix ttm locking to not require this any more,
> and it was needed there only because it was a workaround for the
> locking being wrong.  There was no annotation for the buffer locking
> it was using, so the real problem wasn't easy to spot.

Ah, ok. 

My question really was whether there even was sense for a trylock with
context. I couldn't come up with a case for it; but I think I see one
now.

The thing is; if there could exist something like:

  ww_mutex_trylock(struct ww_mutex *, struct ww_acquire_ctx *ctx);

Then we should not now take away that name and make it mean something
else; namely: ww_mutex_trylock_single().

Unless we want to allow .ctx=NULL to mean _single.

As to why I proposed that (.ctx=NULL meaning _single); I suppose because
I'm a minimalist at heart.
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