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

Tim Chen
2013-09-27T16:19:06Z

On Fri, 2013-09-27 at 09:12 -0700, Jason Low wrote:
> On Fri, 2013-09-27 at 08:02 +0200, Ingo Molnar wrote:
> > Would be nice to have this as a separate, add-on patch. Every single
> > instruction removal that has no downside is an upside!
>
> Okay, so here is a patch. Tim, would you like to add this to v7?

Okay. Will do.

Tim

>
> ...
> Subject: MCS lock: Remove and reorder unnecessary assignments in mcs_spin_lock()
>
> In mcs_spin_lock(), if (likely(prev == NULL)) is true, then the lock is free
> and we won't spin on the local node. In that case, we don't have to assign
> node->locked because it won't be used. We can also move the node->locked = 0
> assignment so that it occurs after the if (likely(prev == NULL)) check.
>
> This might also help make it clearer as to how the node->locked variable
> is used in MCS locks.
>
> Signed-off-by: Jason Low <jason.low2@hp.com>
> ---
> include/linux/mcslock.h | 3 +--
> 1 files changed, 1 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/mcslock.h b/include/linux/mcslock.h
> index 20fd3f0..1167d57 100644
> --- a/include/linux/mcslock.h
> +++ b/include/linux/mcslock.h
> @@ -21,15 +21,14 @@ 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 */
> - node->locked = 1;
> return;
> }
> + node->locked = 0;
> ACCESS_ONCE(prev->next) = node;
> smp_wmb();
> /* Wait until the lock holder passes the lock down */