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=7bvd00iU_0byqmVAe5NoEJ=SwkVbdbcj8+O6=Bh27jzQ@mail.gmail.com>
Date:	Fri, 27 Sep 2013 13:16:16 -0700
From:	Jason Low <jason.low2@...com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Waiman Long <Waiman.Long@...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, Sep 27, 2013 at 12:38 PM, Tim Chen <tim.c.chen@...ux.intel.com> wrote:

> BTW, is the above memory barrier necessary?  It seems like the xchg
> instruction already provided a memory barrier.
>
> Now if we made the changes that Jason suggested:
>
>
>         /* 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();
>
> We are probably still okay as other cpus do not read the value of
> node->locked, which is a local variable.

Similarly, I was wondering if we should also move smp_wmb() so that it
is before the ACCESS_ONCE(prev->next) = node and after the
node->locked = 0. Would we want to guarantee that the node->locked
gets set before it is added to the linked list where a previous thread
calling mcs_spin_unlock() would potentially modify node->locked?
--
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