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Date: Mon, 16 Oct 2017 20:05:30 +0900
From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, Petr Mladek <pmladek@suse.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] printk: Remove superfluous memory barriers from
 printk_safe
Message-ID: <20171016110530.GA6316@tigerII.localdomain>
References: <20171011124647.7781f98f@gandalf.local.home>
 <20171014092129.GD2973@tigerII.localdomain>
 <20171015202715.5c3bb075@vmware.local.home>
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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