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

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Date:   Mon, 16 Oct 2017 20:05:30 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Petr Mladek <pmladek@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] printk: Remove superfluous memory barriers from
 printk_safe

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

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