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Date: Wed, 02 Oct 2013 15:19:29 -0400
From: Waiman Long <waiman.long@hp.com>
MIME-Version: 1.0
Subject: Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and
 locking code into its own file
References: <cover.1380144003.git.tim.c.chen@linux.intel.com>  <1380147049.3467.67.camel@schen9-DESK>  <CAGQ1y=7Ehkr+ot3tDZtHv6FR6RQ9fXBVY0=LOyWjmGH_UjH7xA@mail.gmail.com>  <1380226007.2170.2.camel@buesod1.americas.hpqcorp.net>  <1380226997.2602.11.camel@j-VirtualBox>  <1380228059.2170.10.camel@buesod1.americas.hpqcorp.net>  <1380229794.2602.36.camel@j-VirtualBox>  <1380231702.3467.85.camel@schen9-DESK> <1380235333.3229.39.camel@j-VirtualBox>
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To: Jason Low <jason.low2@hp.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>, Davidlohr Bueso <davidlohr@hp.com>, 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 09/26/2013 06:42 PM, Jason Low wrote:
> On Thu, 2013-09-26 at 14:41 -0700, Tim Chen wrote:
>> Okay, that would makes sense for consistency because we always
>> first set node->lock = 0 at the top of the function.
>>
>> If we prefer to optimize this a bit though, perhaps we can
>> first move the node->lock = 0 so that it gets executed after the
>> "if (likely(prev == NULL)) {}" code block and then delete
>> "node->lock = 1" inside the code block.
>>
>> static noinline
>> void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node *node)
>> {
>>         struct mcs_spin_node *prev;
>>
>>         /* Init node */
>>         node->next   = NULL;
>>
>>         prev = xchg(lock, node);
>>         if (likely(prev == NULL)) {
>>                 /* Lock acquired */
>>                 return;
>>         }
>>         node->locked = 0;

You can remove the locked flag setting statement inside if (prev == 
NULL), but you can't clear the locked flag after xchg(). In the interval 
between xchg() and locked=0, the previous lock owner may come in and set 
the flag. Now if your clear it, the thread will loop forever. You have 
to clear it before xchg().

-Longman

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