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

Petr Mladek
2017-10-16T08:12:55Z

On Sun 2017-10-15 20:27:15, Steven Rostedt wrote:
> On Sat, 14 Oct 2017 18:21:29 +0900
> Sergey Senozhatsky <sergey.senozhatsky@gmail.com> wrote:
>
> > On (10/11/17 12:46), Steven Rostedt wrote:
> > > From: Steven Rostedt (VMware) <rostedt@goodmis.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