linux-kernel - Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file

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Date:	Thu, 26 Sep 2013 13:23:17 -0700
From:	Jason Low <jason.low2@...com>
To:	Davidlohr Bueso <davidlohr@...com>,
	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...aro.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	linux-kernel@...r.kernel.org, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and
 locking code into its own file

On Thu, 2013-09-26 at 13:06 -0700, Davidlohr Bueso wrote:
> 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@...ux.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@...ux.intel.com>
> > > Signed-off-by: Davidlohr Bueso <davidlohr@...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.

If prev == NULL, doesn't that mean it won't proceed to spin on the
variable because the lock is already free and we call return? In that
case where we directly acquire the lock, I was wondering if it is
necessary to set node->locked = 1.

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