Message-ID: <1404411745.8764.27.camel@j-VirtualBox>
Subject: Re: [RFC] Cancellable MCS spinlock rework
From: Jason Low <jason.low2@hp.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@kernel.org>, Waiman Long <Waiman.Long@hp.com>,
        Davidlohr Bueso <davidlohr@hp.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Rik van Riel <riel@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>, Peter Anvin <hpa@zytor.com>,
        Andi Kleen <andi@firstfloor.org>,
        Jej B <James.Bottomley@hansenpartnership.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        "Chandramouleeswaran, Aswin" <aswin@hp.com>,
        "Norton, Scott J" <scott.norton@hp.com>,
        "Vinod, Chegu" <chegu_vinod@hp.com>
Date: Thu, 03 Jul 2014 11:22:25 -0700
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On Thu, 2014-07-03 at 08:35 -0700, Linus Torvalds wrote:
> On Thu, Jul 3, 2014 at 8:31 AM, Linus Torvalds
> <torvalds@linux-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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