x86/dumpstack: Warn on stack recursion

Print a warning if stack recursion is detected.

Use printk_deferred_once() because the unwinder can be called with the
console lock by lockdep via save_stack_trace().

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/def18247aafaab480844484398e793f552b79bda.1477496147.git.jpoimboe@redhat.com
[ Unbroke the lines. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/dumpstack_32.c

@@ -109,8 +109,10 @@ recursion_check:
 	 * just break out and report an unknown stack type.
 	 */
 	if (visit_mask) {
-		if (*visit_mask & (1UL << info->type))
+		if (*visit_mask & (1UL << info->type)) {
+			printk_deferred_once(KERN_WARNING "WARNING: stack recursion on stack type %d\n", info->type);
 			goto unknown;
+		}
 		*visit_mask |= 1UL << info->type;
 	}
 

arch/x86/kernel/dumpstack_64.c

@@ -128,8 +128,10 @@ recursion_check:
 	 * just break out and report an unknown stack type.
 	 */
 	if (visit_mask) {
-		if (*visit_mask & (1UL << info->type))
+		if (*visit_mask & (1UL << info->type)) {
+			printk_deferred_once(KERN_WARNING "WARNING: stack recursion on stack type %d\n", info->type);
 			goto unknown;
+		}
 		*visit_mask |= 1UL << info->type;
 	}