Index: sys/kern/kern_timeout.c
===================================================================
--- sys/kern/kern_timeout.c	(revision 216511)
+++ sys/kern/kern_timeout.c	(working copy)
@@ -115,6 +115,7 @@
 
 #ifdef SMP
 struct callout_cpu cc_cpu[MAXCPU];
+#define	CPUBLOCK	(MAXCPU + 1)
 #define	CC_CPU(cpu)	(&cc_cpu[(cpu)])
 #define	CC_SELF()	CC_CPU(PCPU_GET(cpuid))
 #else
@@ -287,6 +288,12 @@
 
 	for (;;) {
 		cpu = c->c_cpu;
+#ifdef SMP
+		if (cpu == CPUBLOCK) {
+			cpu_spinwait();
+			continue;
+		}
+#endif
 		cc = CC_CPU(cpu);
 		CC_LOCK(cc);
 		if (cpu == c->c_cpu)
@@ -593,7 +600,6 @@
 	 */
 	if (c->c_flags & CALLOUT_LOCAL_ALLOC)
 		cpu = c->c_cpu;
-retry:
 	cc = callout_lock(c);
 	if (cc->cc_curr == c) {
 		/*
@@ -625,15 +631,19 @@
 		cancelled = 1;
 		c->c_flags &= ~(CALLOUT_ACTIVE | CALLOUT_PENDING);
 	}
+#ifdef SMP
 	/*
-	 * If the lock must migrate we have to check the state again as
-	 * we can't hold both the new and old locks simultaneously.
+	 * If the lock must migrate we have to block the callout locking
+	 * until migration is completed.
 	 */
 	if (c->c_cpu != cpu) {
+		c->c_cpu = CPUBLOCK;
+		CC_UNLOCK(cc);
+		cc = CC_CPU(cpu);
+		CC_LOCK(cc);
 		c->c_cpu = cpu;
-		CC_UNLOCK(cc);
-		goto retry;
 	}
+#endif
 
 	if (to_ticks <= 0)
 		to_ticks = 1;