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

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Date:   Thu, 1 Sep 2016 10:17:33 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     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 Thu 2016-09-01 16:55:07, Sergey Senozhatsky wrote:
> 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.

Yes. Well, my concern was how to atomically change the printk_func
pointer and save the previous value at the same time. You could not
use locks because NMIs are involved.

Best Regards,
Petr