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

[Petr Mladek]

On Mon 2019-05-06 11:38:13, Daniel Vetter wrote:
> On Mon, May 06, 2019 at 10:26:28AM +0200, Petr Mladek wrote:
> > 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
> > > variable.
> > 
> > Ah, I was too fast and did the same mistake. This won't help because
> > it would still call try_to_wake_up() recursively.
> 
> Uh :-/
> 
> > 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.
> 
> Hm since it's whack-a-mole anyway, what about converting the WARN_ON into
> a prinkt_deferred, like all the other scheduler related code? Feels a
> notch more consistent to me than leaking the printk_context into areas it
> wasn't really meant built for. Scheduler code already fully subscribed to
> the whack-a-mole approach after all.

I am not sure how exactly you mean the conversion.

Anyway, we do not want to use printk_deferred() treewide. It reduces
the chance that the messages reach consoles. Scheduler is an
exception because of the possible deadlocks.

A solution would be to define WARN_ON_DEFERRED() that would
call normal WARN_ON() in printk deferred context and
use in scheduler.

Best Regards,
Petr