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

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Message-ID: <20171016110845.GB6316@tigerII.localdomain>
Date:   Mon, 16 Oct 2017 20:08:45 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        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 (10/16/17 10:12), Petr Mladek wrote:
[..]
> > > 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.

hm... not so sure, need to check more.

we can access DECLARE_EARLY_PER_CPU() things, which we must copy after
per-cpu pages are set up. `cpu_number' is DECLARE_EARLY_PER_CPU(), btw.

	-ss