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:	Mon, 30 Sep 2013 11:51:47 -0400
From:	Waiman Long <waiman.long@...com>
To:	Jason Low <jason.low2@...com>
CC:	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Tim Chen <tim.c.chen@...ux.intel.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 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.

>> }
>>
>> static void mcs_spin_unlock(struct mcs_spin_node **lock, struct
>> mcs_spin_node *node)
>> {
>>          struct mcs_spin_node *next = ACCESS_ONCE(node->next);
>>
>>          if (likely(!next)) {
>>                  /*
>>                   * Release the lock by setting it to NULL
>>                   */
>>                  if (cmpxchg(lock, node, NULL) == node)
>>                          return;
>>                  /* Wait until the next pointer is set */
>>                  while (!(next = ACCESS_ONCE(node->next)))
>>                          arch_mutex_cpu_relax();
>>          }
>>          smp_wmb();
>>          ACCESS_ONCE(next->locked) = 1;
>> }

Instead, I think what we need may be:

if (likely(!next)) {
     ....
} else
     smp_mb();
ACCESS_ONCE(next->locked) = 1;

That will ensure a memory barrier in the unlock path.

Regards,
Longman
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