linux-kernel - Re: [PATCH] printk/nmi: avoid direct printk()-s from __printk_nmi

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Date:   Thu, 1 Sep 2016 16:55:07 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] printk/nmi: avoid direct printk()-s from
 __printk_nmi_flush()

On (08/30/16 13:19), Petr Mladek wrote:
> 
> I see. But then we will need to be more careful because printk_func
> and printk_func_saved will be manipulated in different contexts:
> normal, irq, nmi. A solution might be using an atomic counter
> and selecting the right vprintk_func according to the value.

alt_printk_enter() must be done with local IRQs disabled. so IRQ cannot
race with `normal' alt_printk. other IRQs cannot race with the current IRQ,
because we have local IRQs disabled. the only thing that can race here is - NMI.
both `normal' and IRQ alt_printk can use the same per-CPU buffer, they never
race. NMI needs to have its own.

	-ss