diff --git a/arch/c5471/src/up_exit.c b/arch/c5471/src/up_exit.c
index c6f544776d..2ef47bdfbc 100644
--- a/arch/c5471/src/up_exit.c
+++ b/arch/c5471/src/up_exit.c
@@ -145,6 +145,13 @@ void _exit(int status)
 
   (void)sched_removereadytorun(tcb);
 
+  /* We are not in a bad stack-- the head of the ready to run task list
+   * does not correspond to the thread that is running.  Disabling pre-
+   * emption on this TCB should be enough to keep things stable.
+   */
+
+  sched_lock();
+
   /* Move the TCB to the specified blocked task list and delete it */
 
   sched_addblocked(tcb, TSTATE_TASK_INACTIVE);
@@ -159,6 +166,10 @@ void _exit(int status)
       (void)sched_mergepending();
     }
 
+  /* Now calling sched_unlock() should have no effect */
+
+  sched_unlock();
+
   /* Now, perform the context switch to the new ready-to-run task at the
    * head of the list.
    */
diff --git a/arch/dm320/src/up_exit.c b/arch/dm320/src/up_exit.c
index e51180dc6d..bb05b1c533 100644
--- a/arch/dm320/src/up_exit.c
+++ b/arch/dm320/src/up_exit.c
@@ -149,6 +149,13 @@ void _exit(int status)
 
   (void)sched_removereadytorun(tcb);
 
+  /* We are not in a bad stack-- the head of the ready to run task list
+   * does not correspond to the thread that is running.  Disabling pre-
+   * emption on this TCB should be enough to keep things stable.
+   */
+
+  sched_lock();
+
   /* Move the TCB to the specified blocked task list and delete it */
 
   sched_addblocked(tcb, TSTATE_TASK_INACTIVE);
@@ -163,6 +170,10 @@ void _exit(int status)
       (void)sched_mergepending();
     }
 
+  /* Now calling sched_unlock() should have no effect */
+
+  sched_unlock();
+
   /* Now, perform the context switch to the new ready-to-run task at the
    * head of the list.
    */
diff --git a/arch/pjrc-8051/src/up_exit.c b/arch/pjrc-8051/src/up_exit.c
index a9b641bab4..b938d93047 100644
--- a/arch/pjrc-8051/src/up_exit.c
+++ b/arch/pjrc-8051/src/up_exit.c
@@ -90,6 +90,13 @@ void _exit(int status)
 
   (void)sched_removereadytorun(tcb);
 
+  /* We are not in a bad stack-- the head of the ready to run task list
+   * does not correspond to the thread that is running.  Disabling pre-
+   * emption on this TCB should be enough to keep things stable.
+   */
+
+  sched_lock();
+
   /* Move the TCB to the specified blocked task list and delete it */
 
   sched_addblocked(tcb, TSTATE_TASK_INACTIVE);
@@ -104,6 +111,10 @@ void _exit(int status)
       (void)sched_mergepending();
     }
 
+  /* Now calling sched_unlock() should have no effect */
+
+  sched_unlock();
+
   /* Now, perform the context switch to the new ready-to-run task at the
    * head of the list.
    */
diff --git a/arch/sim/src/up_exit.c b/arch/sim/src/up_exit.c
index a0a3bdc9ab..0f9c4cc564 100644
--- a/arch/sim/src/up_exit.c
+++ b/arch/sim/src/up_exit.c
@@ -73,6 +73,7 @@
 void _exit(int status)
 {
   _TCB* tcb = (_TCB*)g_readytorun.head;
+  irqstate_t flags;
 
   dbg("TCB=%p exitting\n", tcb);
 
@@ -83,13 +84,20 @@ void _exit(int status)
 
   (void)sched_removereadytorun(tcb);
 
+  /* We are not in a bad stack-- the head of the ready to run task list
+   * does not correspond to the thread that is running.  Disabling pre-
+   * emption on this TCB should be enough to keep things stable.
+   */
+
+  sched_lock();
+
   /* Move the TCB to the specified blocked task list and delete it */
 
   sched_addblocked(tcb, TSTATE_TASK_INACTIVE);
   task_delete(tcb->pid);
 
   /* If there are any pending tasks, then add them to the g_readytorun
-   * task list now
+   * task list now.
    */
 
   if (g_pendingtasks.head)
@@ -97,6 +105,10 @@ void _exit(int status)
       (void)sched_mergepending();
     }
 
+  /* Now calling sched_unlock() should have no effect */
+
+  sched_unlock();
+
   /* Now, perform the context switch to the new ready-to-run task at the
    * head of the list.
    */