Commit 64ca752d authored Jan 21, 2017 by Sergey Senozhatsky Committed by Petr Mladek Mar 24, 2017

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>

parent 7d91de74

kernel/printk/printk.c

+6 −5

Original line number	Diff line number	Diff line
		@@ -2028,14 +2028,15 @@ 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();
		/* If trylock fails, someone else is doing the printing */
		if (console_trylock())
		console_unlock();
		}
		return 0;