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Subject: Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and
 locking code into its own file
From: Davidlohr Bueso <davidlohr@hp.com>
Date: Thu, 26 Sep 2013 13:06:47 -0700
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References: <cover.1380144003.git.tim.c.chen@linux.intel.com>
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To: Jason Low <jason.low2@hp.com>
Cc: Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>, Andrea Arcangeli <aarcange@redhat.com>, Alex Shi <alex.shi@linaro.org>, Andi Kleen <andi@firstfloor.org>, Michel Lespinasse <walken@google.com>, Davidlohr Bueso <davidlohr.bueso@hp.com>, Matthew R Wilcox <matthew.r.wilcox@intel.com>, Dave Hansen <dave.hansen@intel.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>, Rik van Riel <riel@redhat.com>, Peter Hurley <peter@hurleysoftware.com>, linux-kernel@vger.kernel.org, linux-mm <linux-mm@kvack.org>

On Thu, 2013-09-26 at 12:27 -0700, Jason Low wrote:
> On Wed, Sep 25, 2013 at 3:10 PM, Tim Chen <tim.c.chen@linux.intel.com> wrote:
> > We will need the MCS lock code for doing optimistic spinning for rwsem.
> > Extracting the MCS code from mutex.c and put into its own file allow us
> > to reuse this code easily for rwsem.
> >
> > Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> > Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
> > ---
> >  include/linux/mcslock.h |   58 +++++++++++++++++++++++++++++++++++++++++++++++
> >  kernel/mutex.c          |   58 +++++-----------------------------------------
> >  2 files changed, 65 insertions(+), 51 deletions(-)
> >  create mode 100644 include/linux/mcslock.h
> >
> > diff --git a/include/linux/mcslock.h b/include/linux/mcslock.h
> > new file mode 100644
> > index 0000000..20fd3f0
> > --- /dev/null
> > +++ b/include/linux/mcslock.h
> > @@ -0,0 +1,58 @@
> > +/*
> > + * MCS lock defines
> > + *
> > + * This file contains the main data structure and API definitions of MCS lock.
> > + */
> > +#ifndef __LINUX_MCSLOCK_H
> > +#define __LINUX_MCSLOCK_H
> > +
> > +struct mcs_spin_node {
> > +       struct mcs_spin_node *next;
> > +       int               locked;       /* 1 if lock acquired */
> > +};
> > +
> > +/*
> > + * We don't inline mcs_spin_lock() so that perf can correctly account for the
> > + * time spent in this lock function.
> > + */
> > +static noinline
> > +void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node *node)
> > +{
> > +       struct mcs_spin_node *prev;
> > +
> > +       /* Init node */
> > +       node->locked = 0;
> > +       node->next   = NULL;
> > +
> > +       prev = xchg(lock, node);
> > +       if (likely(prev == NULL)) {
> > +               /* Lock acquired */
> > +               node->locked = 1;
> 
> If we don't spin on the local node, is it necessary to set this variable?

I don't follow, the whole idea is to spin on the local variable.

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