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

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Date:   Sat, 14 Oct 2017 18:21:29 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Petr Mladek <pmladek@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH] printk: Remove superfluous memory barriers from
 printk_safe

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?

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

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?

	-ss