Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines andlocking code into its own file

[Jason Low]

On Fri, Sep 27, 2013 at 12:38 PM, Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> wrote:

> BTW, is the above memory barrier necessary?  It seems like the xchg
> instruction already provided a memory barrier.
>
> Now if we made the changes that Jason suggested:
>
>
>         /* Init node */
> -       node->locked = 0;
>         node->next   = NULL;
>
>         prev = xchg(lock, node);
>         if (likely(prev == NULL)) {
>                 /* Lock acquired */
> -               node->locked = 1;
>                 return;
>         }
> +       node->locked = 0;
>         ACCESS_ONCE(prev->next) = node;
>         smp_wmb();
>
> We are probably still okay as other cpus do not read the value of
> node->locked, which is a local variable.

Similarly, I was wondering if we should also move smp_wmb() so that it
is before the ACCESS_ONCE(prev->next) = node and after the
node->locked = 0. Would we want to guarantee that the node->locked
gets set before it is added to the linked list where a previous thread
calling mcs_spin_unlock() would potentially modify node->locked?
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