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

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Message-ID: <20130527111557.GB4341@laptop>
Date:	Mon, 27 May 2013 13:15:57 +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 12:52:00PM +0200, Maarten Lankhorst wrote:
> The reason ttm needed it was because there was another lock that interacted
> with the ctx lock in a weird way. The ww lock it was using was inverted with another
> lock, so it had to grab that lock first, perform a trylock on the ww lock, and if that failed
> unlock the lock, wait for it to be unlocked, then retry the same thing again.
> I'm so glad I managed to fix that mess, if you really need ww_mutex_trylock with a ctx,
> it's an indication your locking is wrong.
> 
> For ww_mutex_trylock with a context to be of any use you would also need to return
> 0 or a -errno, (-EDEADLK, -EBUSY (already locked by someone else), or -EALREADY).
> This would make the trylock very different from other trylocks, and very confusing because
> if (ww_mutex_trylock(lock, ctx)) would not do what you would think it would do.

Yuck ;-)

Anyway, what I was thinking of is something like:

	T0		T1

	try A
			lock B
	lock B
			lock A

Now, if for some reason T1 won the lottery such that T0 would have to be
wounded, T0's context would indicate its the first entry and not return
-EDEADLK.

OTOH, anybody doing creative things like that might well deserve
whatever they get ;-)

> > 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.
> Minimalism isn't bad, it's just knowing when to sto

:-)
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