hung_task: allow hung_task_panic when hung_task_warnings is 0

Previously hung_task_panic would not be respected if enabled after
hung_task_warnings had already been decremented to 0.

Permit the kernel to panic if hung_task_panic is enabled after
hung_task_warnings has already been decremented to 0 and another task
hangs for hung_task_timeout_secs seconds.

Check if hung_task_panic is enabled so we don't return prematurely, and
check if hung_task_warnings is non-zero so we don't print the warning
unnecessarily.

[akpm@linux-foundation.org: fix off-by-one]
Link: http://lkml.kernel.org/r/1473450214-4049-1-git-send-email-jsiddle@redhat.com
Signed-off-by: John Siddle <jsiddle@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/hung_task.c

@@ -98,26 +98,26 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
 
 	trace_sched_process_hang(t);
 
-	if (!sysctl_hung_task_warnings)
+	if (!sysctl_hung_task_warnings && !sysctl_hung_task_panic)
 		return;
 
-	if (sysctl_hung_task_warnings > 0)
-		sysctl_hung_task_warnings--;
-
 	/*
 	 * Ok, the task did not get scheduled for more than 2 minutes,
 	 * complain:
 	 */
-	pr_err("INFO: task %s:%d blocked for more than %ld seconds.\n",
-		t->comm, t->pid, timeout);
-	pr_err("      %s %s %.*s\n",
-		print_tainted(), init_utsname()->release,
-		(int)strcspn(init_utsname()->version, " "),
-		init_utsname()->version);
-	pr_err("\"echo 0 > /proc/sys/kernel/hung_task_timeout_secs\""
-		" disables this message.\n");
-	sched_show_task(t);
-	debug_show_all_locks();
+	if (sysctl_hung_task_warnings) {
+		sysctl_hung_task_warnings--;
+		pr_err("INFO: task %s:%d blocked for more than %ld seconds.\n",
+			t->comm, t->pid, timeout);
+		pr_err("      %s %s %.*s\n",
+			print_tainted(), init_utsname()->release,
+			(int)strcspn(init_utsname()->version, " "),
+			init_utsname()->version);
+		pr_err("\"echo 0 > /proc/sys/kernel/hung_task_timeout_secs\""
+			" disables this message.\n");
+		sched_show_task(t);
+		debug_show_all_locks();
+	}
 
 	touch_nmi_watchdog();