linux-kernel - Re: [PATCH] printk: Remove superfluous memory barriers from printk

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Date:   Mon, 16 Oct 2017 10:12:52 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] printk: Remove superfluous memory barriers from
 printk_safe

On Sun 2017-10-15 20:27:15, Steven Rostedt wrote:
> On Sat, 14 Oct 2017 18:21:29 +0900
> Sergey Senozhatsky <sergey.senozhatsky@...il.com> wrote:
> 
> > On (10/11/17 12:46), Steven Rostedt wrote:
> > > From: Steven Rostedt (VMware) <rostedt@...dmis.org>
> > > 
> > > The variable printk_safe_irq_ready is set and never cleared at system
> > > boot up, when there's only one CPU active. It is set before other
> > > CPUs come on line. Also, it is extremely unlikely that an NMI would
> > > trigger this early in boot up (which I wonder why we even have this
> > > variable at all).  
> > 
> > it's not only NMI related, printk() recursion can happen at any stages,
> > including... um... wait a second. ... including the "before we set up
> > per-CPU areas" stage? hmm... smells like a bug?
> 
> I think this was just being overly paranoid.

I was curious because it was not only about reading the per-CPU
variables. We set and clear the printk_context per-CPU variable
in every printk() call. I wondered if we accessed some
non-initialized stuff.

Fortunately, it seems that we are on the safe side.

If I get it correctly, the per-CPU variables are set up in
setup_per_cpu_areas(). But some per-CPU variables are used even
before, see

  boot_cpu_init()
    smp_processor_id()
      raw_smp_processor_id()
	this_cpu_read(cpu_number)

IMHO, the trick is the following code in setup_per_cpu_areas()
from arch/x86/kernel/setup_percpu.c:

		/*
		 * Up to this point, the boot CPU has been using .init.data
		 * area.  Reload any changed state for the boot CPU.
		 */
		if (!cpu)
			switch_to_new_gdt(cpu);

IMHO, this means that per-CPU variable for the first boot-CPU
can be used at any time. And all the interesting functions:
boot_cpu_init(), setup_per_cpu_areas(), printk_safe_init() are
still called in the single-CPU mode.

Best Regards,
Petr