[External] Re: [PATCH 2/2] sched: mark PRINTK_DEFERRED_CONTEXT_MASK in __schedule()

[Peter Zijlstra]

On Mon, Sep 28, 2020 at 06:04:23PM +0800, Chengming Zhou wrote:

> Well, you are lucky. So it's a problem in our printk implementation.

Not lucky; I just kicked it in the groin really hard:

  git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git debug/experimental

> The deadlock path is:
> 
> printk
>   vprintk_emit
>     console_unlock
>       vt_console_print
>         hide_cursor
>           bit_cursor
>             soft_cursor
>               queue_work_on
>                 __queue_work
>                   try_to_wake_up
>                     _raw_spin_lock
>                       native_queued_spin_lock_slowpath
> 
> Looks like it's introduced by this commit:
> 
> eaa434defaca1781fb2932c685289b610aeb8b4b
> 
> "drm/fb-helper: Add fb_deferred_io support"

Oh gawd, yeah, all the !serial consoles are utter batshit.

Please look at John's last printk rewrite, IIRC it farms all that off to
a kernel thread instead of doing it from the printk() caller's context.

I'm not sure where he hides his latests patches, but I'm sure he'll be
more than happy to tell you.