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

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Date:   Sun, 15 Oct 2017 20:27:15 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     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 Sat, 14 Oct 2017 18:21:29 +0900
Sergey Senozhatsky <sergey.senozhatsky@...il.com> 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.

> 
> do we need to move per-CPU printk_safe buffers out of per-CPU and turn
> it into a global static buffer? like logbuf, and just give every CPU a
> starting offset of its printk_safe_logbuf part.
> 
> IOW,
> 
> char printk_safe_logbuf[number of cpus * sizeof printk safe buffer];
> 
> cpu0 offset 0, up to sizeof printk safe buffer
> cpu1 offset sizeof printk safe buffer, up to 2 * sizeof printk safe buffer

Let's not make this any more complicated than it has to be.

> 
> etc.
> 
> or... at least. avoid stoing to per-CPU printk-safe/printk-nmi buffers
> unless we've got per-CPU areas set up? or am I hallucinating?

We can keep the flag as is, it's highly unlikely to be a problem.

-- Steve