Re: [printk] fbc14616f4: BUG:kernel_reboot-without-warning_in_test_stage

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

On (04/13/17 14:50), Sergey Senozhatsky wrote:
[..]
> On (04/12/17 01:19), Sergey Senozhatsky wrote:
> [..]
> > it does offloading after X printed lines by the same process.
> > if we reschedule, then the counter resets. which is probably OK,
> > we don't really want any process, except for printk_kthread, to
> > stay in console_unlock() forever.
> 
> may be this can be changed. we don't want even printk_kthread to keep
> console_sem locked for too long, because other process that might want
> to lock console_sem have to sleep in TASK_UNINTERRUPTIBLE as long as
> printing thread has pending messages to print. so may be the rule can
> be "every process prints up to `atomic_print_limit' lines and then
> offloads printing - wake_up()s printk_kthread and up()s console_sem".
> some other process (printk_kthread or a process from console_sem wait
> list, let them compete for console_sem) will eventually down()
> console_sem and print the next `atomic_print_limit' lines, while
> current process will have a chance to return from console_unlock() and
> do something else.

something like this, perhaps.

static inline bool console_offload_printing(void)
{
	static struct task_struct *printing_task;
	static unsigned long lines_printed;
	static bool did_wakeup;

	if (!atomic_print_limit || !printk_kthread_enabled())
		return false;

	/* We rescheduled - reset the counters. */
	if (printing_task != current) {
		did_wakeup = false;
		lines_printed = 0;
		printing_task = current;
		return false;
	}

	/*
	 * Don't reset the counter, let CPU overrun the limit.
	 * The idea is that
	 *
	 *   a) woken up printk_kthread (if succeeded)
	 * or
	 *   b) concurrent printk from another CPU (if any)
	 *
	 * will change `printing_task' and reset the counter.
	 * If neither a) nor b) happens - we continue printing from
	 * current process. Which is bad and can be risky, but we can't
	 * wake_up() printk_kthread, so things already don't look normal.
	 */
	lines_printed++;
	if (lines_printed < atomic_print_limit)
		return false;

	if (current == printk_kthread) {
		/*
		 * Reset the counter, just in case if printk_kthread is the
		 * only process left that would down() console_sem.
		 */
		lines_printed = 0;
		return true;
	}

	/*
	 * A trivial emergency enforcement - give up on printk_kthread if
	 * we can't wake it up. This assumes that `atomic_print_limit' is
	 * large enough.
	 */
	if (lines_printed > 2 * (unsigned long long)atomic_print_limit) {
		printk_enforce_emergency = true;
		pr_crit("Declaring printk emergency mode.\n");
		return false;
	}

	/*
	 * Must be executed in 'printk_safe' context. Call into the
	 * scheduler just once, in case if it backfires on us with
	 * warnings and backtraces.
	 */
	if (!did_wakeup) {
		did_wakeup = true;
		wake_up_process(printk_kthread);
	}
	return true;
}

	-ss