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

Jason Low
2013-09-27T16:12:13Z

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?

...
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 */
--
1.7.1