linux-kernel - Re: [RFC] Cancellable MCS spinlock rework

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Message-ID: <1404411745.8764.27.camel@j-VirtualBox>
Date:	Thu, 03 Jul 2014 11:22:25 -0700
From:	Jason Low <jason.low2@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Waiman Long <Waiman.Long@...com>,
	Davidlohr Bueso <davidlohr@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Anvin <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	Jej B <James.Bottomley@...senpartnership.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	"Vinod, Chegu" <chegu_vinod@...com>
Subject: Re: [RFC] Cancellable MCS spinlock rework

On Thu, 2014-07-03 at 08:35 -0700, Linus Torvalds wrote:
> On Thu, Jul 3, 2014 at 8:31 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > I don't think we'll support 2 _billion_ processes/threads waiting on
> > the same semaphore any time soon, so the 'long' seems a bit of an
> > overkill on 64-bit architectures.
> 
> Oh, never mind. The 'struct semaphore' uses it as just a plain count,
> but the rwsem ends up splitting up the bits for readers/writers, so we
> actually do want the full 64-bit value there.

Yeah, that was the key to reducing the struct size as the rwsem count
can't share a 64 bit chunk with the spinlock, unlike some of the other
lock.

Thanks,
Jason

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