Messages in this thread Patch in this message | | | Subject | Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file | From | Jason Low <> | Date | Fri, 27 Sep 2013 09:12:03 -0700 |
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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
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