linux-kernel - Re: [PATCH 1/2] printk: Introduce LOUD

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Message-ID: <ZxoGriUNSepndV77@pathway.suse.cz>
Date: Thu, 24 Oct 2024 10:34:54 +0200
From: Petr Mladek <pmladek@...e.com>
To: Marcos Paulo de Souza <mpdesouza@...e.com>
Cc: John Ogness <john.ogness@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Subject: Re: [PATCH 1/2] printk: Introduce LOUD_CON flag

On Wed 2024-10-23 17:36:04, Marcos Paulo de Souza wrote:
> On Mon, 2024-10-21 at 16:17 +0206, John Ogness wrote:
> > On 2024-10-21, Petr Mladek <pmladek@...e.com> wrote:
> > > > That will not work because migrate_enable() can only be called
> > > > from
> > > > can_sleep context. Instead, the migrate_disable()/enable() should
> > > > be at
> > > > the few (one?) call sites where
> > > > printk_loud_console_enter()/exit() is
> > > > used from task context.
> > > 
> > > Hmm, if I get it correctly, we could not use migrate_disable() in
> > > __handle_sysrq() because it can be called also in atomic context,
> > 
> > I am talking about callers of __handle_sysrq() and/or their callers.
> > 
> > For example write_sysrq_trigger() could do:
> > 
> > 	migrate_disable();
> > 	__handle_sysrq(c, false);
> > 	migrate_enable();
> > 
> > Or a new wrapper could be introduced for this purpose:
> > 
> > static inline void wrapper handle_sysrq_task(u8 key, bool check_mask)
> > {
> > 	migrate_disable();
> > 	__handle_sysrq(key, check_mask);
> > 	migrate_enable();
> > }
> > 
> > A quick grep shows about 25 call sites to check.
> >
> > > I do not see any easy way how to distinguish whether it was called
> > > in
> > > an atomic context or not.
> > 
> > There is no clean way to do that. If this information is needed, it
> > must
> > be tracked by the call chain.
> > 
> > > So, I see three possibilities:
> > > 
> > >   1. Explicitly call preempt_disable() in __handle_sysrq().
> > > 
> > >      It would be just around the the single line or the help. But
> > > still,
> > >      I do not like it much.
> > 
> > Not acceptable for PREEMPT_RT since sysrq is exposed to external
> > inputs.
> > 
> > >   2. Avoid the per-CPU variable. Force adding the
> > > LOUD_CON/FORCE_CON
> > >      flag using a global variable, e.g. printk_force_console.
> > > 
> > >      The problem is that it might affect also messages printed by
> > >      other CPUs. And there might be many.
> > > 
> > >      Well, console_loglevel is a global variable. The original code
> > >      had a similar problem.
> > 
> > If disabling migration is not an option for you, this would be my
> > second
> > choice. I assume tagging too many messages is better than not tagging
> > enough. And, as you say, this is effectively what the current code is
> > trying to do.
> 
> Thanks for your input John. I talked with Petr and he suggested to
> follow this option. I'll prepare the changes and send them after
> reviewing them with Petr.

Just for record. I propose this way because disabling migration would
require checking all callers (25 as mentioned above).
migrate_disable() is needed and can be called only in the task
context.

I do not think that it is worth the effort. And it would be error
prone (hard to maintain).

Best Regards,
Petr