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:	Wed, 2 Oct 2013 12:30:00 -0700
From:	Jason Low <jason.low2@...com>
To:	Waiman Long <waiman.long@...com>
Cc:	Jason Low <jason.low2@...com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	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 Wed, Oct 2, 2013 at 12:19 PM, Waiman Long <waiman.long@...com> wrote:
> 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().

Yes, in my most recent version, I left locked = 0 in its original
place so that the xchg() can act as a barrier for it.

The other option would have been to put another barrier after locked =
0. I went with leaving locked = 0 in its original place so that we
don't need that extra barrier.
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