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:	Fri, 27 Sep 2013 09:19:01 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Jason Low <jason.low2@...com>
Cc:	Ingo Molnar <mingo@...nel.org>, Davidlohr Bueso <davidlohr@...com>,
	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 Fri, 2013-09-27 at 09:12 -0700, Jason Low wrote:
> On Fri, 2013-09-27 at 08:02 +0200, Ingo Molnar wrote:
> > Would be nice to have this as a separate, add-on patch. Every single 
> > instruction removal that has no downside is an upside!
> 
> Okay, so here is a patch. Tim, would you like to add this to v7?

Okay.  Will do.

Tim

> 
> ...
> Subject: MCS lock: Remove and reorder unnecessary assignments in mcs_spin_lock()
> 
> In mcs_spin_lock(), if (likely(prev == NULL)) is true, then the lock is free
> and we won't spin on the local node. In that case, we don't have to assign
> node->locked because it won't be used. We can also move the node->locked = 0
> assignment so that it occurs after the if (likely(prev == NULL)) check.
> 
> This might also help make it clearer as to how the node->locked variable
> is used in MCS locks.
> 
> Signed-off-by: Jason Low <jason.low2@...com>
> ---
>  include/linux/mcslock.h |    3 +--
>  1 files changed, 1 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mcslock.h b/include/linux/mcslock.h
> index 20fd3f0..1167d57 100644
> --- a/include/linux/mcslock.h
> +++ b/include/linux/mcslock.h
> @@ -21,15 +21,14 @@ 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;
>  		return;
>  	}
> +	node->locked = 0;
>  	ACCESS_ONCE(prev->next) = node;
>  	smp_wmb();
>  	/* Wait until the lock holder passes the lock down */


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