Index: sys/amd64/amd64/vm_machdep.c
===================================================================
--- sys/amd64/amd64/vm_machdep.c
+++ sys/amd64/amd64/vm_machdep.c
@@ -484,13 +484,12 @@
 }
 
 /*
- * Set that machine state for performing an upcall that has to
- * be done in thread_userret() so that those upcalls generated
- * in thread_userret() itself can be done as well.
+ * Set that machine state for performing an upcall that starts
+ * the entry function with the given argument.
  */
 void
 cpu_set_upcall_kse(struct thread *td, void (*entry)(void *), void *arg,
-	stack_t *stack)
+    stack_t *stack)
 {
 
 	/* 
@@ -505,7 +504,7 @@
 #ifdef COMPAT_FREEBSD32
 	if (SV_PROC_FLAG(td->td_proc, SV_ILP32)) {
 		/*
-	 	 * Set the trap frame to point at the beginning of the uts
+		 * Set the trap frame to point at the beginning of the entry
 		 * function.
 		 */
 		td->td_frame->tf_rbp = 0;
@@ -513,10 +512,7 @@
 		   (((uintptr_t)stack->ss_sp + stack->ss_size - 4) & ~0x0f) - 4;
 		td->td_frame->tf_rip = (uintptr_t)entry;
 
-		/*
-		 * Pass the address of the mailbox for this kse to the uts
-		 * function as a parameter on the stack.
-		 */
+		/* Pass the argument to the entry point. */
 		suword32((void *)(td->td_frame->tf_rsp + sizeof(int32_t)),
 		    (uint32_t)(uintptr_t)arg);
 
@@ -539,10 +535,7 @@
 	td->td_frame->tf_gs = _ugssel;
 	td->td_frame->tf_flags = TF_HASSEGS;
 
-	/*
-	 * Pass the address of the mailbox for this kse to the uts
-	 * function as a parameter on the stack.
-	 */
+	/* Pass the argument to the entry point. */
 	td->td_frame->tf_rdi = (register_t)arg;
 }
 
Index: sys/i386/i386/vm_machdep.c
===================================================================
--- sys/i386/i386/vm_machdep.c
+++ sys/i386/i386/vm_machdep.c
@@ -527,13 +527,12 @@
 }
 
 /*
- * Set that machine state for performing an upcall that has to
- * be done in thread_userret() so that those upcalls generated
- * in thread_userret() itself can be done as well.
+ * Set that machine state for performing an upcall that starts
+ * the entry function with the given argument.
  */
 void
 cpu_set_upcall_kse(struct thread *td, void (*entry)(void *), void *arg,
-	stack_t *stack)
+    stack_t *stack)
 {
 
 	/* 
@@ -546,7 +545,7 @@
 	cpu_thread_clean(td);
 
 	/*
-	 * Set the trap frame to point at the beginning of the uts
+	 * Set the trap frame to point at the beginning of the entry
 	 * function.
 	 */
 	td->td_frame->tf_ebp = 0; 
@@ -554,10 +553,7 @@
 	    (((int)stack->ss_sp + stack->ss_size - 4) & ~0x0f) - 4;
 	td->td_frame->tf_eip = (int)entry;
 
-	/*
-	 * Pass the address of the mailbox for this kse to the uts
-	 * function as a parameter on the stack.
-	 */
+	/* Pass the argument to the entry point. */
 	suword((void *)(td->td_frame->tf_esp + sizeof(void *)),
 	    (int)arg);
 }
Index: sys/kern/kern_thread.c
===================================================================
--- sys/kern/kern_thread.c
+++ sys/kern/kern_thread.c
@@ -481,7 +481,7 @@
 	 * architecture specific resources that
 	 * would not be on a new untouched process.
 	 */
-	cpu_thread_exit(td);	/* XXXSMP */
+	cpu_thread_exit(td);
 
 	/*
 	 * The last thread is left attached to the process
@@ -670,11 +670,6 @@
 			wakeup_swapper |= sleepq_abort(td2, EINTR);
 		break;
 	case SINGLE_BOUNDARY:
-		if (TD_IS_SUSPENDED(td2) && (td2->td_flags & TDF_BOUNDARY) == 0)
-			wakeup_swapper |= thread_unsuspend_one(td2, p, false);
-		if (TD_ON_SLEEPQ(td2) && (td2->td_flags & TDF_SINTR) != 0)
-			wakeup_swapper |= sleepq_abort(td2, ERESTART);
-		break;
 	case SINGLE_NO_EXIT:
 		if (TD_IS_SUSPENDED(td2) && (td2->td_flags & TDF_BOUNDARY) == 0)
 			wakeup_swapper |= thread_unsuspend_one(td2, p, false);
@@ -913,8 +908,8 @@
 			/*
 			 * The only suspension in action is a
 			 * single-threading. Single threader need not stop.
-			 * XXX Should be safe to access unlocked
-			 * as it can only be set to be true by us.
+			 * It is safe to access p->p_singlethread unlocked
+			 * because it can only be set to our address by us.
 			 */
 			if (p->p_singlethread == td)
 				return (0);	/* Exempt from stopping. */