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:	Fri, 27 Sep 2013 21:34:06 -0700
From:	Jason Low <jason.low2@...com>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	Jason Low <jason.low2@...com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Waiman Long <Waiman.Long@...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 Fri, Sep 27, 2013 at 7:19 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Fri, Sep 27, 2013 at 04:54:06PM -0700, Jason Low wrote:
>> On Fri, Sep 27, 2013 at 4:01 PM, Paul E. McKenney
>> <paulmck@...ux.vnet.ibm.com> wrote:
>> > Yep.  The previous lock holder's smp_wmb() won't keep either the compiler
>> > or the CPU from reordering things for the new lock holder.  They could for
>> > example reorder the critical section to precede the node->locked check,
>> > which would be very bad.
>>
>> Paul, Tim, Longman,
>>
>> How would you like the proposed changes below?
>
> Could you point me at what this applies to?  I can find flaws looking
> at random pieces, given a little luck, but at some point I need to look
> at the whole thing.  ;-)

Sure. Here is a link to the patch we are trying to modify:
https://lkml.org/lkml/2013/9/25/532

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();
}

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;
}
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