linux-kernel - Re: [PATCH 2/3] early_printk: Add force_early

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Date:   Fri, 13 Oct 2017 15:06:09 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     sergey.senozhatsky@...il.com, linux-kernel@...r.kernel.org,
        rostedt@...dmis.org, mingo@...nel.org, tglx@...utronix.de
Subject: Re: [PATCH 2/3] early_printk: Add force_early_printk kernel parameter

On Thu 2017-10-12 13:39:49, Peter Zijlstra wrote:
> On Thu, Oct 12, 2017 at 12:24:19PM +0200, Petr Mladek wrote:
> > On Thu 2017-09-28 14:18:25, Peter Zijlstra wrote:
> 
> > > +#ifdef CONFIG_EARLY_PRINTK
> > > +struct console *early_console;
> > > +
> > > +static bool __read_mostly force_early_printk;
> > > +
> > > +static int __init force_early_printk_setup(char *str)
> > > +{
> > > +	force_early_printk = true;
> > > +	return 0;
> > > +}
> > > +early_param("force_early_printk", force_early_printk_setup);
> > 
> > The parameter is currently used only when CONFIG_PRINTK is enabled.
> > But CONFIG_EARLY_PRINTK is independent. What would be your preferred
> > behavior when CONFIG_PRINTK is disabled, please?
> 
> Can we even have !PRINTK && EARLY_PRINTK? If so it seems to me continued
> usage of early_printk() is what makes most sense.

Yes, !PRINTK && EARLY_PRINTK is possible at the moment. It makes some
sense because EARLY_PRINTK needs only consoles and they are needed
also for !PRINTK stuff.

We either should define force_early_printk only when
PRINTK is enabled.

Or we should call early_printk() from printk() also when
PRINTK is disabled. The current implemetation is in
include/linux/printk.h, see

static inline __printf(1, 2) __cold
int printk(const char *s, ...)
{
	return 0;
}

> > > @@ -1816,6 +1852,11 @@ asmlinkage int vprintk_emit(int facility
> > >  		return vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args);
> > >  #endif
> > >  
> > > +#ifdef CONFIG_EARLY_PRINTK
> > > +	if (force_early_printk && early_console)
> > > +		return early_vprintk(fmt, args);
> > > +#endif
> > > +
> > >  	if (level == LOGLEVEL_SCHED) {
> > >  		level = LOGLEVEL_DEFAULT;
> > >  		in_sched = true;
> > > @@ -1939,7 +1980,12 @@ asmlinkage __visible int printk(const ch
> > >  	int r;
> > >  
> > >  	va_start(args, fmt);
> > > -	r = vprintk_func(fmt, args);
> > > +#ifdef CONFIG_EARLY_PRINTK
> > > +	if (force_early_printk && early_console)
> > > +		r = vprintk_default(fmt, args);
> > > +	else
> > > +#endif
> > > +		r = vprintk_func(fmt, args);
> > 
> > There is rather theoretical race. We skip vprintk_func() because
> > we believe that vprintk_default()/vprintk_emit() would choose
> > handle this by early_printk().
> 
> Do you mean if someone were to toggle force_early_printk at runtime?

Or that someone unregisters the early console.


> The reason I did it like this and not use that function pointer thing is
> that I didn't want to risk anybody hijacking my output ever.

I understand. I think about refactoring the code so that all
*printk*() variants call printk_func(). This function
could then call the right printk implementation according
to the context or global setting.

This way we could have all the logic on a single place and
avoid the race.

Note that printk_func() is not longer a pointer. It is
a function since the commit 099f1c84c0052ec1b2
("printk: introduce per-cpu safe_print seq buffer").

Best Regards,
Petr