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: <524C74C3.4060908@hp.com>
Date:	Wed, 02 Oct 2013 15:32:19 -0400
From:	Waiman Long <waiman.long@...com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
CC:	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Jason Low <jason.low2@...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 10/02/2013 02:43 PM, Tim Chen wrote:
> On Tue, 2013-10-01 at 21:25 -0400, Waiman Long wrote:
>>
>> If the lock and unlock functions are done right, there should be no
>> overlap of critical section. So it is job of the lock/unlock functions
>> to make sure that critical section code won't leak out. There should be
>> some kind of memory barrier at the beginning of the lock function and
>> the end of the unlock function.
>>
>> The critical section also likely to have branches. The CPU may
>> speculatively execute code on the 2 branches, but one of them will be
>> discarded once the branch condition is known. Also
>> arch_mutex_cpu_relax() is a compiler barrier by itself. So we may not
>> need a barrier() after all. The while statement is a branch instruction,
>> any code after that can only be speculatively executed and cannot be
>> committed until the branch is done.
> But the condition code may be checked after speculative execution?
> The condition may not be true during speculative execution and only
> turns true when we check the condition, and take that branch?
>
> The thing that bothers me is without memory barrier after the while
> statement, we could speculatively execute before affirming the lock is
> in acquired state. Then when we check the lock, the lock is set
> to acquired state in the mean time.
> We could be loading some memory entry *before*
> the node->locked has been set true.  I think a smp_rmb (if not a
> smp_mb) should be set after the while statement.

Yes, I think a smp_rmb() make sense here to correspond to the smp_wmb() 
in the unlock path.

BTW, you need to move the node->locked = 0; statement before xchg() if 
you haven't done so.

-Longman


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