Re: [PATCH] RFC: console: hack up console_lock more v2

From: Petr Mladek
Date: Mon May 06 2019 - 04:21:01 EST

On Mon 2019-05-06 09:45:53, Daniel Vetter wrote:
> console_trylock, called from within printk, can be called from pretty
> much anywhere. Including try_to_wake_up. Note that this isn't common,
> usually the box is in pretty bad shape at that point already. But it
> really doesn't help when then lockdep jumps in and spams the logs,
> potentially obscuring the real backtrace we're really interested in.
> One case I've seen (slightly simplified backtrace):
> Call Trace:
> <IRQ>
> console_trylock+0xe/0x60
> vprintk_emit+0xf1/0x320
> printk+0x4d/0x69
> __warn_printk+0x46/0x90
> native_smp_send_reschedule+0x2f/0x40
> check_preempt_curr+0x81/0xa0
> ttwu_do_wakeup+0x14/0x220
> try_to_wake_up+0x218/0x5f0

try_to_wake_up() takes p->pi_lock. It could deadlock because it
can get called recursively from printk_safe_up().

And there are more locks taken from try_to_wake_up(), for example,
__task_rq_lock() taken from ttwu_remote().

IMHO, the most reliable solution would be do call the entire
up_console_sem() from printk deferred context. We could assign
few bytes for this context in the per-CPU printk_deferred

Best Regards,