Re: [PATCH] RFC: console: hack up console_lock more v2
From: Petr Mladek
Date: Mon May 06 2019 - 04:27:10 EST
On Mon 2019-05-06 10:16:14, Petr Mladek wrote:
> 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
Ah, I was too fast and did the same mistake. This won't help because
it would still call try_to_wake_up() recursively.
We need to call all printk's that can be called under locks
taken in try_to_wake_up() path in printk deferred context.
Unfortunately it is whack a mole approach.