linux-kernel - Re: [RFC] printk/sysrq: Don't play with console

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Date:   Wed, 12 Jun 2019 15:47:38 +0100
From:   Dmitry Safonov <dima@...sta.com>
To:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Subject: Re: [RFC] printk/sysrq: Don't play with console_loglevel

On 6/12/19 1:00 PM, Petr Mladek wrote:
> On Wed 2019-06-12 17:36:43, Sergey Senozhatsky wrote:
>> On (06/06/19 09:10), Petr Mladek wrote:
>>> Just to be sure. I wanted to say that I like the idea with
>>> KERN_UNSUPRESSED. So, I think that we are on the same page.
>>
>> I understand. All I wanted to say is that KERN_UNSUPRESSED is
>> per-message, while the most interesting (and actually broken)
>> cases, IMHO, are per-context, IOW things like this one
>>
>> 	console_loglevel = NEW
>> 	foo()
>> 	  dump_stack()
>> 	     printk
>> 	     ...
>> 	     printk
>> 	console_loglevel = OLD
>>
>> KERN_UNSUPRESSED does not help here. We probably can't convert
>> dump_stack() to KERN_UNSUPRESSED.
> 
> I agree. I take KERN_UNSUPRESSED like a nice trick how to pass
> the information about the unsupressed printk context via
> printk_safe and printk_nmi per-CPU buffers.
> 
> The single line in sysrq __handle_sysrq() seems to be the only
> location where KERN_UNSUPRESSED can be used directly.

I likely don't understand all the reasons why it's not possible.

Looking at kdb - prints those can't be converted straight-away are
show_regs() and show_stack().. could we add helpers those take a
loglevel to show the info? (having as an example show_trace_log_lvl()
that can "eat" a modifier already).

Not that I want to sell the idea of KERN_UNSUPRESSED, but to my mind the
alternative patches were kind of too intricate and bring more complexity
to printk.. and there are only ~3(?) places those manipulate
console_loglevel directly in the kernel tree (so we might want something
simple and dumb as hell).

Thanks,
          Dima