linux-kernel - Re: [PATCH v2] debugobjects: Move printk out of db lock critical sections

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Date:   Tue, 18 Dec 2018 15:06:03 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ingo Molnar <mingo@...nel.org>
cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Waiman Long <longman@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        yang.shi@...ux.alibaba.com, Arnd Bergmann <arnd@...db.de>,
        sergey.senozhatsky.work@...il.com, dima@...sta.com, bp@...en8.de
Subject: Re: [PATCH v2] debugobjects: Move printk out of db lock critical
 sections

On Tue, 18 Dec 2018, Ingo Molnar wrote:
> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> > So thinking that early_printk is any better is just puting your head in 
> > the sand.
> 
> ... at my own feet. ;-) Apologies to the syslog folks!
> 
> early_printk should still in principle be more robust: it tries to use as 
> little (no) locking as possible, and definitely tries to do no 
> allocations. It doesn't use syslog, nor any console locking, nor any 
> regular console drivers.
> 
> Which results in usability trade-offs: trashed screen output, mangled 
> lines. It's a superior debug facility when debugging particularly hairy 
> low level code - which most of the kernel isn't where it turns into an 
> inferior debugging method.
> 
> I thing a good solution would be PeterZ's force_early_printk boot knob, 
> for those low level folks who absolutely want to rely on printk always 
> working in some fashion.
> 
> ( I think it might even be possible to add a non-locked feature to 
>   early-printk that actually adds the messages to the syslog ring-buffer 
>   - without any notification/wakeup/serialization features. 'dmesg' is 
>   handy and its lack is the primary usability disadvantage of 
>   earlyprintk. )

There is work in progress on that... Should be in your inbox early next
year.

Thanks,

	tglx