From 8562e5b08027692b9a31552d59055ac5acd15e68 Mon Sep 17 00:00:00 2001
From: jasone <jasone@FreeBSD.org>
Date: Sun, 30 Nov 2008 05:55:24 +0000
Subject: Do not spin when trying to lock on a single-CPU system.

Reported by:	davidxu
---
 lib/libc/stdlib/malloc.c | 24 +++++++++++++-----------
 1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/lib/libc/stdlib/malloc.c b/lib/libc/stdlib/malloc.c
index 55bc413..4b37e72 100644
--- a/lib/libc/stdlib/malloc.c
+++ b/lib/libc/stdlib/malloc.c
@@ -1262,18 +1262,20 @@ malloc_spin_lock(pthread_mutex_t *lock)
 
 	if (__isthreaded) {
 		if (_pthread_mutex_trylock(lock) != 0) {
-			unsigned i;
-			volatile unsigned j;
-
-			/* Exponentially back off. */
-			for (i = 1; i <= SPIN_LIMIT_2POW; i++) {
-				for (j = 0; j < (1U << i); j++) {
-					ret++;
-					CPU_SPINWAIT;
+			/* Exponentially back off if there are multiple CPUs. */
+			if (ncpus > 1) {
+				unsigned i;
+				volatile unsigned j;
+
+				for (i = 1; i <= SPIN_LIMIT_2POW; i++) {
+					for (j = 0; j < (1U << i); j++) {
+						ret++;
+						CPU_SPINWAIT;
+					}
+
+					if (_pthread_mutex_trylock(lock) == 0)
+						return (ret);
 				}
-
-				if (_pthread_mutex_trylock(lock) == 0)
-					return (ret);
 			}
 
 			/*
-- 
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