Index: head/sys/kern/kern_linker.c
===================================================================
--- head/sys/kern/kern_linker.c
+++ head/sys/kern/kern_linker.c
@@ -1021,15 +1021,33 @@
  * obey locking protocols, and offer a significantly less complex interface.
  */
 int
-linker_search_symbol_name(caddr_t value, char *buf, u_int buflen,
-    long *offset)
+linker_search_symbol_name_flags(caddr_t value, char *buf, u_int buflen,
+    long *offset, int flags)
 {
 	int error;
 
-	sx_slock(&kld_sx);
+	KASSERT((flags & (M_NOWAIT | M_WAITOK)) != 0 &&
+	    (flags & (M_NOWAIT | M_WAITOK)) != (M_NOWAIT | M_WAITOK),
+	    ("%s: bad flags: 0x%x", __func__, flags));
+
+	if (flags & M_NOWAIT) {
+		if (!sx_try_slock(&kld_sx))
+			return (EWOULDBLOCK);
+	} else
+		sx_slock(&kld_sx);
+
 	error = linker_debug_search_symbol_name(value, buf, buflen, offset);
 	sx_sunlock(&kld_sx);
 	return (error);
+}
+
+int
+linker_search_symbol_name(caddr_t value, char *buf, u_int buflen,
+    long *offset)
+{
+
+	return (linker_search_symbol_name_flags(value, buf, buflen, offset,
+	    M_WAITOK));
 }
 
 /*
Index: head/sys/kern/subr_stack.c
===================================================================
--- head/sys/kern/subr_stack.c
+++ head/sys/kern/subr_stack.c
@@ -48,7 +48,7 @@
 static MALLOC_DEFINE(M_STACK, "stack", "Stack Traces");
 
 static int stack_symbol(vm_offset_t pc, char *namebuf, u_int buflen,
-	    long *offset);
+	    long *offset, int flags);
 static int stack_symbol_ddb(vm_offset_t pc, const char **name, long *offset);
 
 struct stack *
@@ -102,7 +102,7 @@
 	KASSERT(st->depth <= STACK_MAX, ("bogus stack"));
 	for (i = 0; i < st->depth; i++) {
 		(void)stack_symbol(st->pcs[i], namebuf, sizeof(namebuf),
-		    &offset);
+		    &offset, M_WAITOK);
 		printf("#%d %p at %s+%#lx\n", i, (void *)st->pcs[i],
 		    namebuf, offset);
 	}
@@ -120,7 +120,7 @@
 		if (i > 0)
 			printf(" ");
 		if (stack_symbol(st->pcs[i], namebuf, sizeof(namebuf),
-		    &offset) == 0)
+		    &offset, M_WAITOK) == 0)
 			printf("%s+%#lx", namebuf, offset);
 		else
 			printf("%p", (void *)st->pcs[i]);
@@ -165,25 +165,36 @@
 #endif
 
 /*
- * Two print routines -- one for use from DDB and DDB-like contexts, the
- * other for use in the live kernel.
+ * Format stack into sbuf from live kernel.
+ *
+ * flags - M_WAITOK or M_NOWAIT (EWOULDBLOCK).
  */
-void
-stack_sbuf_print(struct sbuf *sb, const struct stack *st)
+int
+stack_sbuf_print_flags(struct sbuf *sb, const struct stack *st, int flags)
 {
 	char namebuf[64];
 	long offset;
-	int i;
+	int i, error;
 
 	KASSERT(st->depth <= STACK_MAX, ("bogus stack"));
 	for (i = 0; i < st->depth; i++) {
-		(void)stack_symbol(st->pcs[i], namebuf, sizeof(namebuf),
-		    &offset);
+		error = stack_symbol(st->pcs[i], namebuf, sizeof(namebuf),
+		    &offset, flags);
+		if (error == EWOULDBLOCK)
+			return (error);
 		sbuf_printf(sb, "#%d %p at %s+%#lx\n", i, (void *)st->pcs[i],
 		    namebuf, offset);
 	}
+	return (0);
 }
 
+void
+stack_sbuf_print(struct sbuf *sb, const struct stack *st)
+{
+
+	(void)stack_sbuf_print_flags(sb, st, M_WAITOK);
+}
+
 #if defined(DDB) || defined(WITNESS)
 void
 stack_sbuf_print_ddb(struct sbuf *sb, const struct stack *st)
@@ -246,16 +257,19 @@
  * and bypasses linker locking, and the other that doesn't.
  */
 static int
-stack_symbol(vm_offset_t pc, char *namebuf, u_int buflen, long *offset)
+stack_symbol(vm_offset_t pc, char *namebuf, u_int buflen, long *offset,
+    int flags)
 {
+	int error;
 
-	if (linker_search_symbol_name((caddr_t)pc, namebuf, buflen,
-	    offset) != 0) {
-		*offset = 0;
-		strlcpy(namebuf, "??", buflen);
-		return (ENOENT);
-	} else
-		return (0);
+	error = linker_search_symbol_name_flags((caddr_t)pc, namebuf, buflen,
+	    offset, flags);
+	if (error == 0 || error == EWOULDBLOCK)
+		return (error);
+
+	*offset = 0;
+	strlcpy(namebuf, "??", buflen);
+	return (ENOENT);
 }
 
 static int
Index: head/sys/sys/linker.h
===================================================================
--- head/sys/sys/linker.h
+++ head/sys/sys/linker.h
@@ -183,6 +183,8 @@
 /*
  * stack(9) helper for situations where kernel locking is required.
  */
+int linker_search_symbol_name_flags(caddr_t value, char *buf, u_int buflen,
+    long *offset, int flags);
 int linker_search_symbol_name(caddr_t value, char *buf, u_int buflen,
     long *offset);
 
Index: head/sys/sys/stack.h
===================================================================
--- head/sys/sys/stack.h
+++ head/sys/sys/stack.h
@@ -47,6 +47,8 @@
 void		 stack_print_short_ddb(const struct stack *);
 void		 stack_sbuf_print(struct sbuf *, const struct stack *);
 void		 stack_sbuf_print_ddb(struct sbuf *, const struct stack *);
+int		 stack_sbuf_print_flags(struct sbuf *, const struct stack *,
+		 int);
 #ifdef KTR
 void		 stack_ktr(u_int, const char *, int, const struct stack *,
 		    u_int, int);