Re: [PATCH] printk: Remove superfluous memory barriers from printk_safe

Sergey Senozhatsky
2017-10-16T11:08:52Z

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