printk: use console_trylock() in console_cpu_notify()

There is no need to always call blocking console_lock() in
console_cpu_notify(), it's quite possible that console_sem can
be locked by other CPU on the system, either already printing
or soon to begin printing the messages. console_lock() in this
case can simply block CPU hotplug for unknown period of time
(console_unlock() is time unbound). Not that hotplug is very
fast, but still, with other CPUs being online and doing
printk() console_cpu_notify() can stuck.

Use console_trylock() instead and opt-out if console_sem is
already acquired from another CPU, since that CPU will do
the printing for us.

Link: http://lkml.kernel.org/r/20170121104729.8585-1-sergey.senozhatsky@gmail.com
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

kernel/printk/printk.c

@@ -2028,15 +2028,16 @@ void resume_console(void)
  * @cpu: unused
  *
  * If printk() is called from a CPU that is not online yet, the messages
- * will be spooled but will not show up on the console.  This function is
- * called when a new CPU comes online (or fails to come up), and ensures
- * that any such output gets printed.
+ * will be printed on the console only if there are CON_ANYTIME consoles.
+ * This function is called when a new CPU comes online (or fails to come
+ * up) or goes offline.
  */
 static int console_cpu_notify(unsigned int cpu)
 {
 	if (!cpuhp_tasks_frozen) {
-		console_lock();
-		console_unlock();
+		/* If trylock fails, someone else is doing the printing */
+		if (console_trylock())
+			console_unlock();
 	}
 	return 0;
 }