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

[Waiman Long]

On 09/26/2013 06:42 PM, Jason Low wrote:
> On Thu, 2013-09-26 at 14:41 -0700, Tim Chen wrote:
> > Okay, that would makes sense for consistency because we always
> > first set node->lock = 0 at the top of the function.
> >
> > If we prefer to optimize this a bit though, perhaps we can
> > first move the node->lock = 0 so that it gets executed after the
> > "if (likely(prev == NULL)) {}" code block and then delete
> > "node->lock = 1" inside the code block.
> >
> > static noinline
> > void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node *node)
> > {
> >         struct mcs_spin_node *prev;
> >
> >         /* Init node */
> >         node->next   = NULL;
> >
> >         prev = xchg(lock, node);
> >         if (likely(prev == NULL)) {
> >                 /* Lock acquired */
> >                 return;
> >         }
> >         node->locked = 0;

You can remove the locked flag setting statement inside if (prev == 
NULL), but you can't clear the locked flag after xchg(). In the interval 
between xchg() and locked=0, the previous lock owner may come in and set 
the flag. Now if your clear it, the thread will loop forever. You have 
to clear it before xchg().

-Longman
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/