linux-kernel - Re: [PATCH v11 0/4] Introducing a queue read/write lock implementation

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Message-ID: <20140130181136.GP7575@mudshark.cambridge.arm.com>
Date:	Thu, 30 Jan 2014 18:11:36 +0000
From:	Will Deacon <will.deacon@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Waiman Long <Waiman.Long@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	George Spelvin <linux@...izon.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	"Aswin Chandramouleeswaran\"" <aswin@...com>,
	Scott J Norton <scott.norton@...com>,
	"will@...ldeacon.co.uk" <will@...ldeacon.co.uk>
Subject: Re: [PATCH v11 0/4] Introducing a queue read/write lock
 implementation

On Thu, Jan 30, 2014 at 06:05:33PM +0000, Peter Zijlstra wrote:
> On Thu, Jan 30, 2014 at 05:52:12PM +0000, Will Deacon wrote:
> > It would be nice if these were default implementations of the unlock, then
> > architectures just implement atomic_sub_release how they like.
> 
> Yes, I suppose that makes sense. Last time I proposed the primitive
> nobody yelled at me, so I suppose that means people agree :-)

If it's useful for these qrwlocks, that's good enough for me!

> > One thing worth mentioning: I have a fairly invasive set of changes pending
> > for arch/arm64/include/asm/atomic.h, so if you do decide to go with this,
> > I'm more than happy to take the sub_release part via the arm64 tree. I guess
> > it depends on when this is likely to get merged.
> 
> I suppose it depends on when I get enough courage to do: vim
> arch/*/include/asm/atomic*.h :-)

Hehe.

> There's a few other cleanups I want to do, like today I found
> atomic_{set,clear}_mask() instead of the more natural atomic_{or,and}()
> functions.

Have you looked at the OpenCL atomic intrinsics at all?

  http://www.khronos.org/registry/cl/sdk/1.2/docs/man/xhtml/atomicFunctions.html

There's a good chance that they can be implemented efficiently on any
architectures that care about OpenCL. As you've noticed, composing them
together can be more efficient on LL/SC-based architectures too.

> I also think we can get rid of the {inc,dec} variants of
> smp_mb__{before,after}_atomic() since these barriers should be the same
> for _all_ atomic ops that do not already imply full mb semantics, and
> they're certainly the same for all current inc/dec.

Makes sense.

> If tomorrow is another slow day and I get through enough of the review
> backlog I might just give it a go.
> 
> Anyway, I'll base them on your arm64 changes, I know where to find
> those.

Okey doke. If you need a stable (non-rebasing) branch, just holler.

Cheers,

Will
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