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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Message-ID: <CAGQ1y=7Ehkr+ot3tDZtHv6FR6RQ9fXBVY0=LOyWjmGH_UjH7xA@mail.gmail.com>
Date:	Thu, 26 Sep 2013 12:27:23 -0700
From:	Jason Low <jason.low2@...com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	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 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?

> +               return;
> +       }
> +       ACCESS_ONCE(prev->next) = node;
> +       smp_wmb();
> +       /* Wait until the lock holder passes the lock down */
> +       while (!ACCESS_ONCE(node->locked))
> +               arch_mutex_cpu_relax();
> +}
> +
> +static void mcs_spin_unlock(struct mcs_spin_node **lock, struct mcs_spin_node *node)
> +{
> +       struct mcs_spin_node *next = ACCESS_ONCE(node->next);
> +
> +       if (likely(!next)) {
> +               /*
> +                * Release the lock by setting it to NULL
> +                */
> +               if (cmpxchg(lock, node, NULL) == node)

And can we make this check likely()?
--
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