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

Sergey Senozhatsky
2017-10-16T11:05:35Z

On (10/15/17 20:27), Steven Rostedt 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.

hm, printk recursion is pretty easy to trigger. vscnprintf() can WARN_ON(),
for instance. just pass (mistakenly) unknown printk specifier, e.g.
%A, and this will do.

-ss