diff --git a/sys/alpha/alpha/trap.c b/sys/alpha/alpha/trap.c
index bbd1d5a82e2f..9c2ea34f7f7e 100644
--- a/sys/alpha/alpha/trap.c
+++ b/sys/alpha/alpha/trap.c
@@ -297,13 +297,6 @@ trap(a0, a1, a2, entry, framep)
 	if (user)  {
 		sticks = td->td_kse->ke_sticks;
 		td->td_frame = framep;
-#ifdef DIAGNOSTIC
-		/* see the comment in ast() */
-		if (td->td_ucred != NULL)
-			panic("trap(): thread got a ucred while in userspace");
-		td->td_ucred = td->td_ucred_cache;
-		td->td_ucred_cache = NULL;
-#endif
 		if (td->td_ucred != p->p_ucred)
 			cred_update_thread(td);
 	} else {
@@ -631,12 +624,9 @@ trap(a0, a1, a2, entry, framep)
 		framep->tf_regs[FRAME_SP] = alpha_pal_rdusp();
 		userret(td, framep, sticks);
 		mtx_assert(&Giant, MA_NOTOWNED);
-#ifdef DIAGNOSTIC 			/* see the comment in ast() */
-		if (td->td_ucred_cache)
-			panic("trap: thread already has cached ucred");
-		td->td_ucred_cache = td->td_ucred;
-		td->td_ucred = NULL;
-#endif /* DIAGNOSTIC */
+#ifdef DIAGNOSTIC
+		cred_free_thread(td);
+#endif
 	}
 	return;
 
@@ -705,12 +695,6 @@ syscall(code, framep)
 	td->td_frame = framep;
 	opc = framep->tf_regs[FRAME_PC] - 4;
 	sticks = td->td_kse->ke_sticks;
-#ifdef DIAGNOSTIC 			/* see the comment in ast() */
-	if (td->td_ucred)
-		panic("syscall:thread got a cred while in userspace");
-	td->td_ucred = td->td_ucred_cache;
-	td->td_ucred_cache = NULL;
-#endif /* DIAGNOSTIC */
 	if (td->td_ucred != p->p_ucred)
 		cred_update_thread(td);
 
@@ -833,12 +817,9 @@ syscall(code, framep)
 	 */
 	STOPEVENT(p, S_SCX, code);
 
-#ifdef DIAGNOSTIC 			/* see the comment in ast() */
-	if (td->td_ucred_cache)
-		panic("syscall:thread already has cached ucred");
-	td->td_ucred_cache = td->td_ucred;
-	td->td_ucred = NULL;
-#endif /* DIAGNOSTIC */
+#ifdef DIAGNOSTIC
+	cred_free_thread(td);
+#endif
 #ifdef WITNESS
 	if (witness_list(td)) {
 		panic("system call %s returning with mutex(s) held\n",
diff --git a/sys/amd64/amd64/trap.c b/sys/amd64/amd64/trap.c
index 57d1dea71c5d..751d5f575160 100644
--- a/sys/amd64/amd64/trap.c
+++ b/sys/amd64/amd64/trap.c
@@ -257,13 +257,6 @@ trap(frame)
 
 		sticks = td->td_kse->ke_sticks;
 		td->td_frame = &frame;
-#ifdef DIAGNOSTIC
-		/* see the comment in ast() */
-		if (td->td_ucred != NULL)
-			panic("trap(): thread got a ucred while in userspace");
-		td->td_ucred = td->td_ucred_cache;
-		td->td_ucred_cache = NULL;
-#endif
 		if (td->td_ucred != p->p_ucred) 
 			cred_update_thread(td);
 
@@ -651,12 +644,9 @@ trap(frame)
 	userret(td, &frame, sticks);
 	mtx_assert(&Giant, MA_NOTOWNED);
 userout:
-#ifdef DIAGNOSTIC 			/* see the comment in ast() */
-	if (td->td_ucred_cache)
-		panic("trap:thread already has cached ucred");
-	td->td_ucred_cache = td->td_ucred;
-        td->td_ucred = NULL;
-#endif /* DIAGNOSTIC */
+#ifdef DIAGNOSTIC
+	cred_free_thread(td);
+#endif
 out:
 	return;
 }
@@ -964,12 +954,6 @@ syscall(frame)
 
 	sticks = td->td_kse->ke_sticks;
 	td->td_frame = &frame;
-#ifdef DIAGNOSTIC 			/* see the comment in ast() */
-	if (td->td_ucred)
-		panic("trap:thread got a cred while userspace");
-	td->td_ucred = td->td_ucred_cache;
-	td->td_ucred_cache = NULL;
-#endif /* DIAGNOSTIC */
 	if (td->td_ucred != p->p_ucred) 
 		cred_update_thread(td);
 	params = (caddr_t)frame.tf_esp + sizeof(int);
@@ -1114,12 +1098,9 @@ syscall(frame)
 	 */
 	STOPEVENT(p, S_SCX, code);
 
-#ifdef DIAGNOSTIC 			/* see the comment in ast() */
-	if (td->td_ucred_cache)
-		panic("syscall:thread already has cached ucred");
-	td->td_ucred_cache = td->td_ucred;
-        td->td_ucred = NULL;
-#endif /* DIAGNOSTIC */
+#ifdef DIAGNOSTIC
+	cred_free_thread(td);
+#endif
 
 #ifdef WITNESS
 	if (witness_list(td)) {
diff --git a/sys/i386/i386/trap.c b/sys/i386/i386/trap.c
index 57d1dea71c5d..751d5f575160 100644
--- a/sys/i386/i386/trap.c
+++ b/sys/i386/i386/trap.c
@@ -257,13 +257,6 @@ trap(frame)
 
 		sticks = td->td_kse->ke_sticks;
 		td->td_frame = &frame;
-#ifdef DIAGNOSTIC
-		/* see the comment in ast() */
-		if (td->td_ucred != NULL)
-			panic("trap(): thread got a ucred while in userspace");
-		td->td_ucred = td->td_ucred_cache;
-		td->td_ucred_cache = NULL;
-#endif
 		if (td->td_ucred != p->p_ucred) 
 			cred_update_thread(td);
 
@@ -651,12 +644,9 @@ trap(frame)
 	userret(td, &frame, sticks);
 	mtx_assert(&Giant, MA_NOTOWNED);
 userout:
-#ifdef DIAGNOSTIC 			/* see the comment in ast() */
-	if (td->td_ucred_cache)
-		panic("trap:thread already has cached ucred");
-	td->td_ucred_cache = td->td_ucred;
-        td->td_ucred = NULL;
-#endif /* DIAGNOSTIC */
+#ifdef DIAGNOSTIC
+	cred_free_thread(td);
+#endif
 out:
 	return;
 }
@@ -964,12 +954,6 @@ syscall(frame)
 
 	sticks = td->td_kse->ke_sticks;
 	td->td_frame = &frame;
-#ifdef DIAGNOSTIC 			/* see the comment in ast() */
-	if (td->td_ucred)
-		panic("trap:thread got a cred while userspace");
-	td->td_ucred = td->td_ucred_cache;
-	td->td_ucred_cache = NULL;
-#endif /* DIAGNOSTIC */
 	if (td->td_ucred != p->p_ucred) 
 		cred_update_thread(td);
 	params = (caddr_t)frame.tf_esp + sizeof(int);
@@ -1114,12 +1098,9 @@ syscall(frame)
 	 */
 	STOPEVENT(p, S_SCX, code);
 
-#ifdef DIAGNOSTIC 			/* see the comment in ast() */
-	if (td->td_ucred_cache)
-		panic("syscall:thread already has cached ucred");
-	td->td_ucred_cache = td->td_ucred;
-        td->td_ucred = NULL;
-#endif /* DIAGNOSTIC */
+#ifdef DIAGNOSTIC
+	cred_free_thread(td);
+#endif
 
 #ifdef WITNESS
 	if (witness_list(td)) {
diff --git a/sys/ia64/ia64/trap.c b/sys/ia64/ia64/trap.c
index 18204351fa1a..060dcb5a3b5c 100644
--- a/sys/ia64/ia64/trap.c
+++ b/sys/ia64/ia64/trap.c
@@ -323,13 +323,6 @@ trap(int vector, int imm, struct trapframe *framep)
 	if (user) {
 		sticks = td->td_kse->ke_sticks;
 		td->td_frame = framep;
-#ifdef DIAGNOSTIC
-		/* see the comment in ast() */
-		if (td->td_ucred != NULL)
-			panic("trap(): thread got a ucred while in userspace");
-		td->td_ucred = td->td_ucred_cache;
-		td->td_ucred_cache = NULL;
-#endif
 		if (td->td_ucred != p->p_ucred)
 			cred_update_thread(td);
 	} else {
@@ -714,12 +707,9 @@ trap(int vector, int imm, struct trapframe *framep)
 	if (user) {
 		userret(td, framep, sticks);
 		mtx_assert(&Giant, MA_NOTOWNED);
-#ifdef DIAGNOSTIC 			/* see the comment in ast() */
-		if (td->td_ucred_cache)
-			panic("trap:thread already has cached ucred");
-		td->td_ucred_cache = td->td_ucred;
-        	td->td_ucred = NULL;
-#endif /* DIAGNOSTIC */
+#ifdef DIAGNOSTIC
+		cred_free_thread(td);
+#endif
 	}
 	return;
 
@@ -764,12 +754,6 @@ syscall(int code, u_int64_t *args, struct trapframe *framep)
 
 	td->td_frame = framep;
 	sticks = td->td_kse->ke_sticks;
-#ifdef DIAGNOSTIC 			/* see the comment in ast() */
-	if (td->td_ucred)
-		panic("trap:thread got a cred while userspace");
-	td->td_ucred = td->td_ucred_cache;
-	td->td_ucred_cache = NULL;
-#endif /* DIAGNOSTIC */
 	if (td->td_ucred != p->p_ucred)
 		cred_update_thread(td);
 
@@ -876,12 +860,9 @@ syscall(int code, u_int64_t *args, struct trapframe *framep)
 	 */
 	STOPEVENT(p, S_SCX, code);
 
-#ifdef DIAGNOSTIC 			/* see the comment in ast() */
-	if (td->td_ucred_cache)
-		panic("trap:thread already has cached ucred");
-	td->td_ucred_cache = td->td_ucred;
-       	td->td_ucred = NULL;
-#endif /* DIAGNOSTIC */
+#ifdef DIAGNOSTIC
+	cred_free_thread(td);
+#endif
 #ifdef WITNESS
 	if (witness_list(td)) {
 		panic("system call %s returning with mutex(s) held\n",
diff --git a/sys/powerpc/aim/trap.c b/sys/powerpc/aim/trap.c
index 16ab7ae5717f..6c122e69e665 100644
--- a/sys/powerpc/aim/trap.c
+++ b/sys/powerpc/aim/trap.c
@@ -226,13 +226,6 @@ trap(struct trapframe *frame)
 	if (user) {
 		sticks = td->td_kse->ke_sticks;
 		td->td_frame = frame;
-#ifdef DIAGNOSTIC
-		/* see the comment in ast() */
-		if (td->td_ucred != NULL)
-			panic("trap(): thread got a ucred while in userspace");
-		td->td_ucred = td->td_ucred_cache;
-		td->td_ucred_cache = NULL;
-#endif
 		if (td->td_ucred != p->p_ucred)
 			cred_update_thread(td);
 
@@ -303,12 +296,9 @@ trap(struct trapframe *frame)
 	}
 	userret(td, frame, sticks);
 	mtx_assert(&Giant, MA_NOTOWNED);
-#ifdef DIAGNOSTIC 			/* see the comment in ast() */
-	if (td->td_ucred_cache)
-		panic("trap:thread already has cached ucred");
-	td->td_ucred_cache = td->td_ucred;
-       	td->td_ucred = NULL;
-#endif /* DIAGNOSTIC */
+#ifdef DIAGNOSTIC
+	cred_free_thread(td);
+#endif
 }
 
 void
diff --git a/sys/powerpc/powerpc/trap.c b/sys/powerpc/powerpc/trap.c
index 16ab7ae5717f..6c122e69e665 100644
--- a/sys/powerpc/powerpc/trap.c
+++ b/sys/powerpc/powerpc/trap.c
@@ -226,13 +226,6 @@ trap(struct trapframe *frame)
 	if (user) {
 		sticks = td->td_kse->ke_sticks;
 		td->td_frame = frame;
-#ifdef DIAGNOSTIC
-		/* see the comment in ast() */
-		if (td->td_ucred != NULL)
-			panic("trap(): thread got a ucred while in userspace");
-		td->td_ucred = td->td_ucred_cache;
-		td->td_ucred_cache = NULL;
-#endif
 		if (td->td_ucred != p->p_ucred)
 			cred_update_thread(td);
 
@@ -303,12 +296,9 @@ trap(struct trapframe *frame)
 	}
 	userret(td, frame, sticks);
 	mtx_assert(&Giant, MA_NOTOWNED);
-#ifdef DIAGNOSTIC 			/* see the comment in ast() */
-	if (td->td_ucred_cache)
-		panic("trap:thread already has cached ucred");
-	td->td_ucred_cache = td->td_ucred;
-       	td->td_ucred = NULL;
-#endif /* DIAGNOSTIC */
+#ifdef DIAGNOSTIC
+	cred_free_thread(td);
+#endif
 }
 
 void
diff --git a/sys/sparc64/sparc64/trap.c b/sys/sparc64/sparc64/trap.c
index 481cdab7692f..f2eeaa6ae093 100644
--- a/sys/sparc64/sparc64/trap.c
+++ b/sys/sparc64/sparc64/trap.c
@@ -177,13 +177,6 @@ trap(struct trapframe *tf)
 	if ((type & T_KERNEL) == 0) {
 		sticks = td->td_kse->ke_sticks;
 		td->td_frame = tf;
-#ifdef DIAGNOSTIC
-		/* see the comment in ast() */
-		if (td->td_ucred != NULL)
-			panic("trap(): thread got a ucred while in userspace");
-		td->td_ucred = td->td_ucred_cache;
-		td->td_ucred_cache = NULL;
-#endif
 		if (td->td_ucred != p->p_ucred)
 			cred_update_thread(td);
  	} else {
@@ -387,12 +380,9 @@ if ((type & ~T_KERNEL) != T_BREAKPOINT)
 user:
 	userret(td, tf, sticks);
 	mtx_assert(&Giant, MA_NOTOWNED);
-#ifdef DIAGNOSTIC 			/* see the comment in ast() */
-	if (td->td_ucred_cache)
-		panic("trap:thread already has cached ucred");
-	td->td_ucred_cache = td->td_ucred;
-       	td->td_ucred = NULL;
-#endif /* DIAGNOSTIC */
+#ifdef DIAGNOSTIC
+	cred_free_thread(td);
+#endif
 out:
 	CTR1(KTR_TRAP, "trap: td=%p return", td);
 	return;
@@ -548,12 +538,6 @@ syscall(struct trapframe *tf)
 
 	sticks = td->td_kse->ke_sticks;
 	td->td_frame = tf;
-#ifdef DIAGNOSTIC 			/* see the comment in ast() */
-	if (td->td_ucred)
-		panic("syscall:thread got a cred while userspace");
-	td->td_ucred = td->td_ucred_cache;
-	td->td_ucred_cache = NULL;
-#endif /* DIAGNOSTIC */
 	if (td->td_ucred != p->p_ucred)
 		cred_update_thread(td);
 	code = tf->tf_global[1];
@@ -690,12 +674,9 @@ syscall(struct trapframe *tf)
 	 */
 	STOPEVENT(p, S_SCX, code);
 
-#ifdef DIAGNOSTIC 			/* see the comment in ast() */
-	if (td->td_ucred_cache)
-		panic("syscall:thread already has cached ucred");
-	td->td_ucred_cache = td->td_ucred;
-       	td->td_ucred = NULL;
-#endif /* DIAGNOSTIC */
+#ifdef DIAGNOSTIC
+	cred_free_thread(td);
+#endif
 #ifdef WITNESS
 	if (witness_list(td)) {
 		panic("system call %s returning with mutex(s) held\n",
diff --git a/sys/sys/proc.h b/sys/sys/proc.h
index 58940f16e1b0..72656e8ddf1c 100644
--- a/sys/sys/proc.h
+++ b/sys/sys/proc.h
@@ -289,11 +289,6 @@ struct thread {
 #define	td_endcopy td_pcb
 
 	struct ucred	*td_ucred;	/* (k) Reference to credentials. */
-#ifdef DIAGNOSTIC 			/* see the comment in ast() */
-	struct ucred	*td_ucred_cache; /* (k) hide cred here for DIAGNOSTIC */
-#else
-	void 		*td_dontuse;	/* keep the size the same if not DIAG */
-#endif
 	struct pcb	*td_pcb;	/* (k) Kernel VA of pcb and kstack. */
 	struct callout	td_slpcallout;	/* (h) Callout for sleep. */
 	struct trapframe *td_frame;	/* (k) */