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:	Tue, 01 Oct 2013 16:01:05 -0400
From:	Waiman Long <waiman.long@...com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
CC:	Jason Low <jason.low2@...com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.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 10/01/2013 12:48 PM, Tim Chen wrote:
> On Mon, 2013-09-30 at 12:36 -0400, Waiman Long wrote:
>> On 09/30/2013 12:10 PM, Jason Low wrote:
>>> On Mon, 2013-09-30 at 11:51 -0400, Waiman Long wrote:
>>>> On 09/28/2013 12:34 AM, Jason Low wrote:
>>>>>> Also, below is what the mcs_spin_lock() and mcs_spin_unlock()
>>>>>> functions would look like after applying the proposed changes.
>>>>>>
>>>>>> 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. No need to set node->locked since it
>>>>>> won't be used */
>>>>>>                    return;
>>>>>>            }
>>>>>>            ACCESS_ONCE(prev->next) = node;
>>>>>>            /* Wait until the lock holder passes the lock down */
>>>>>>            while (!ACCESS_ONCE(node->locked))
>>>>>>                    arch_mutex_cpu_relax();
>>>>>>            smp_mb();
>>>> I wonder if a memory barrier is really needed here.
>>> If the compiler can reorder the while (!ACCESS_ONCE(node->locked)) check
>>> so that the check occurs after an instruction in the critical section,
>>> then the barrier may be necessary.
>>>
>> In that case, just a barrier() call should be enough.
> The cpu could still be executing out of order load instruction from the
> critical section before checking node->locked?  Probably smp_mb() is
> still needed.
>
> Tim

But this is the lock function, a barrier() call should be enough to 
prevent the critical section from creeping up there. We certainly need 
some kind of memory barrier at the end of the unlock function.

-Longman
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