diff --git a/sys/libkern/arc4random.c b/sys/libkern/arc4random.c
index 909d750b8532..3a3227cd211f 100644
--- a/sys/libkern/arc4random.c
+++ b/sys/libkern/arc4random.c
@@ -1,143 +1,152 @@
 /*-
  * THE BEER-WARE LICENSE
  *
  * <dan@FreeBSD.ORG> wrote this file.  As long as you retain this notice you
  * can do whatever you want with this stuff.  If we meet some day, and you
  * think this stuff is worth it, you can buy me a beer in return.
  *
  * Dan Moschuk
  */
 
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
 #include <sys/types.h>
+#include <sys/param.h>
+#include <sys/kernel.h>
 #include <sys/random.h>
 #include <sys/libkern.h>
+#include <sys/lock.h>
+#include <sys/mutex.h>
 #include <sys/time.h>
 
 #define	ARC4_RESEED_BYTES 65536
 #define	ARC4_RESEED_SECONDS 300
 #define	ARC4_KEYBYTES (256 / 8)
 
 static u_int8_t arc4_i, arc4_j;
-static int arc4_initialized = 0;
 static int arc4_numruns = 0;
 static u_int8_t arc4_sbox[256];
 static time_t arc4_t_reseed;
+static struct mtx arc4_mtx;
 
 static u_int8_t arc4_randbyte(void);
 
 static __inline void
 arc4_swap(u_int8_t *a, u_int8_t *b)
 {
 	u_int8_t c;
 
 	c = *a;
 	*a = *b;
 	*b = c;
 }	
 
 /*
  * Stir our S-box.
  */
 static void
 arc4_randomstir (void)
 {
 	u_int8_t key[256];
 	int r, n;
 	struct timeval tv_now;
 
 	/*
 	 * XXX read_random() returns unsafe numbers if the entropy
 	 * device is not loaded -- MarkM.
 	 */
 	r = read_random(key, ARC4_KEYBYTES);
+	getmicrouptime(&tv_now);
+	mtx_lock(&arc4_mtx);
 	/* If r == 0 || -1, just use what was on the stack. */
 	if (r > 0) {
 		for (n = r; n < sizeof(key); n++)
 			key[n] = key[n % r];
 	}
 
 	for (n = 0; n < 256; n++) {
 		arc4_j = (arc4_j + arc4_sbox[n] + key[n]) % 256;
 		arc4_swap(&arc4_sbox[n], &arc4_sbox[arc4_j]);
 	}
 
 	/* Reset for next reseed cycle. */
-	getmicrouptime(&tv_now);
 	arc4_t_reseed = tv_now.tv_sec + ARC4_RESEED_SECONDS;
 	arc4_numruns = 0;
+
+	/*
+	 * Throw away the first N words of output, as suggested in the
+	 * paper "Weaknesses in the Key Scheduling Algorithm of RC4"
+	 * by Fluher, Mantin, and Shamir.  (N = 256 in our case.)
+	 */
+	for (n = 0; n < 256*4; n++)
+		arc4_randbyte();
+	mtx_unlock(&arc4_mtx);
 }
 
 /*
  * Initialize our S-box to its beginning defaults.
  */
 static void
 arc4_init(void)
 {
 	int n;
 
+	mtx_init(&arc4_mtx, "arc4_mtx", NULL, MTX_DEF);
 	arc4_i = arc4_j = 0;
 	for (n = 0; n < 256; n++)
 		arc4_sbox[n] = (u_int8_t) n;
 
-	arc4_randomstir();
-	arc4_initialized = 1;
-
-	/*
-	 * Throw away the first N words of output, as suggested in the
-	 * paper "Weaknesses in the Key Scheduling Algorithm of RC4"
-	 * by Fluher, Mantin, and Shamir.  (N = 256 in our case.)
-	 */
-	for (n = 0; n < 256*4; n++)
-		arc4_randbyte();
+	arc4_t_reseed = 0;
 }
 
+SYSINIT(arc4_init, SI_SUB_LOCK, SI_ORDER_ANY, arc4_init, NULL);
+
 /*
  * Generate a random byte.
  */
 static u_int8_t
 arc4_randbyte(void)
 {
 	u_int8_t arc4_t;
 
 	arc4_i = (arc4_i + 1) % 256;
 	arc4_j = (arc4_j + arc4_sbox[arc4_i]) % 256;
 
 	arc4_swap(&arc4_sbox[arc4_i], &arc4_sbox[arc4_j]);
 
 	arc4_t = (arc4_sbox[arc4_i] + arc4_sbox[arc4_j]) % 256;
 	return arc4_sbox[arc4_t];
 }
 
+/*
+ * MPSAFE
+ */
 void
 arc4rand(void *ptr, u_int len, int reseed)
 {
 	u_char *p;
 	struct timeval tv;
 
-	/* Initialize array if needed. */
-	if (!arc4_initialized)
-		arc4_init();
-
 	getmicrouptime(&tv);
-	arc4_numruns += len;
 	if (reseed || 
 	   (arc4_numruns > ARC4_RESEED_BYTES) ||
 	   (tv.tv_sec > arc4_t_reseed))
 		arc4_randomstir();
 
+	mtx_lock(&arc4_mtx);
+	arc4_numruns += len;
 	p = ptr;
 	while (len--)
 		*p++ = arc4_randbyte();
+	mtx_unlock(&arc4_mtx);
 }
 
 uint32_t
 arc4random(void)
 {
 	uint32_t ret;
 
 	arc4rand(&ret, sizeof ret, 0);
 	return ret;
 }