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Subject[PATCH v8 6/9] MCS Lock: optimizations and extra comments
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Remove unnecessary operation and make the cmpxchg(lock, node, NULL) == node
check in mcs_spin_unlock() likely() as it is likely that a race did not occur
most of the time.

Also add in more comments describing how the local node is used in MCS locks.

Signed-off-by: Jason Low <jason.low2@hp.com>
---
include/linux/mcs_spinlock.h | 13 +++++++++++--
1 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/include/linux/mcs_spinlock.h b/include/linux/mcs_spinlock.h
index b5de3b0..96f14299 100644
--- a/include/linux/mcs_spinlock.h
+++ b/include/linux/mcs_spinlock.h
@@ -18,6 +18,12 @@ struct mcs_spinlock {
};

/*
+ * In order to acquire the lock, the caller should declare a local node and
+ * pass a reference of the node to this function in addition to the lock.
+ * If the lock has already been acquired, then this will proceed to spin
+ * on this node->locked until the previous lock holder sets the node->locked
+ * in mcs_spin_unlock().
+ *
* We don't inline mcs_spin_lock() so that perf can correctly account for the
* time spent in this lock function.
*/
@@ -33,7 +39,6 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
prev = xchg(lock, node);
if (likely(prev == NULL)) {
/* Lock acquired */
- node->locked = 1;
return;
}
ACCESS_ONCE(prev->next) = node;
@@ -43,6 +48,10 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
arch_mutex_cpu_relax();
}

+/*
+ * Releases the lock. The caller should pass in the corresponding node that
+ * was used to acquire the lock.
+ */
static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
{
struct mcs_spinlock *next = ACCESS_ONCE(node->next);
@@ -51,7 +60,7 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod
/*
* Release the lock by setting it to NULL
*/
- if (cmpxchg(lock, node, NULL) == node)
+ if (likely(cmpxchg(lock, node, NULL) == node))
return;
/* Wait until the next pointer is set */
while (!(next = ACCESS_ONCE(node->next)))
--
1.7.4.4




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