From e5c830a47d6178217ddc1e192e4c28a9c2857ec2 Mon Sep 17 00:00:00 2001
From: Matthew Dillon <dillon@dragonflybsd.org>
Date: Mon, 29 May 2006 16:50:06 +0000
Subject: [PATCH] Shortcut two common spinlock situations and don't bother KTR
 logging them.

---
 sys/kern/kern_spinlock.c | 13 ++++++++++++-
 sys/sys/spinlock2.h      | 12 ++++++++----
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/sys/kern/kern_spinlock.c b/sys/kern/kern_spinlock.c
index a1be17636a..13f4206948 100644
--- a/sys/kern/kern_spinlock.c
+++ b/sys/kern/kern_spinlock.c
@@ -29,7 +29,7 @@
  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  *
- * $DragonFly: src/sys/kern/kern_spinlock.c,v 1.5 2006/05/29 07:29:14 dillon Exp $
+ * $DragonFly: src/sys/kern/kern_spinlock.c,v 1.6 2006/05/29 16:50:05 dillon Exp $
  */
 
 #include <sys/param.h>
@@ -200,8 +200,19 @@ spin_lock_rd_contested(struct spinlock *mtx)
 	globaldata_t gd = mycpu;
 	int value = mtx->lock;
 
+	/*
+	 * Shortcut the op if we can just set the cache bit.  This case
+	 * occurs when the last lock was an exclusive lock.
+	 */
+	while ((value & SPINLOCK_EXCLUSIVE) == 0) {
+		if (atomic_cmpset_int(&mtx->lock, value, value|gd->gd_cpumask))
+			return;
+		value = mtx->lock;
+	}
+
 	exponential_init(&backoff, mtx);
 	++spinlocks_contested1;
+
 	logspin(beg, mtx, 'r');
 
 	while ((value & gd->gd_cpumask) == 0) {
diff --git a/sys/sys/spinlock2.h b/sys/sys/spinlock2.h
index 5a4f30d307..8e599655f9 100644
--- a/sys/sys/spinlock2.h
+++ b/sys/sys/spinlock2.h
@@ -29,7 +29,7 @@
  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  *
- * $DragonFly: src/sys/sys/spinlock2.h,v 1.9 2006/05/21 20:23:27 dillon Exp $
+ * $DragonFly: src/sys/sys/spinlock2.h,v 1.10 2006/05/29 16:50:06 dillon Exp $
  */
 
 #ifndef _SYS_SPINLOCK2_H_
@@ -92,7 +92,8 @@ spin_trylock_wr(struct spinlock *mtx)
 #endif
 
 /*
- * Obtain an exclusive spinlock and return.
+ * Obtain an exclusive spinlock and return.  Shortcut the case where the only
+ * cached read lock was from our own cpu (it can just be cleared).
  */
 static __inline void
 spin_lock_wr_quick(globaldata_t gd, struct spinlock *mtx)
@@ -103,8 +104,11 @@ spin_lock_wr_quick(globaldata_t gd, struct spinlock *mtx)
 
 	++gd->gd_spinlocks_wr;
 #ifdef SMP
-	if ((value = atomic_swap_int(&mtx->lock, SPINLOCK_EXCLUSIVE)) != 0)
-		spin_lock_wr_contested(mtx, value);
+	if ((value = atomic_swap_int(&mtx->lock, SPINLOCK_EXCLUSIVE)) != 0) {
+		value &= ~gd->gd_cpumask;
+		if (value)
+			spin_lock_wr_contested(mtx, value);
+	}
 #endif
 }
 
-- 
2.41.0