linux-kernel - Re: [PATCH v2] printk/nmi: avoid direct printk()-s from __printk_nmi

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Date:   Thu, 1 Sep 2016 09:55:53 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>, Jan Kara <jack@...e.cz>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] printk/nmi: avoid direct printk()-s from
 __printk_nmi_flush()

On (08/31/16 13:15), Andrew Morton wrote:
> > On (08/30/16 15:03), Andrew Morton wrote:
> > > > __printk_nmi_flush() can be called from nmi_panic(), therefore it has to
> > > > test whether it's executed in NMI context and thus must route the messages
> > > > through deferred printk() or via direct printk().
> > > 
> > > Why?  What misbehaviour does the current code cause?
> > 
> > the reasoning behind the `if in_nmi()' in print_nmi_seq_line()
> > 
> >        if (in_nmi())
> >                printk_deferred("%.*s", (end - start) + 1, buf);
> >        else
> >                printk("%.*s", (end - start) + 1, buf);
> > 
> > was as follows (per Petr's commit message)
> 
> OK, thanks, I altered the changelog thusly and scheduled the patch for 4.8:

thanks!

	-ss

> 
> --- txt/printk-nmi-avoid-direct-printk-s-from-__printk_nmi_flush.txt
> +++ txt/printk-nmi-avoid-direct-printk-s-from-__printk_nmi_flush.txt
> @@ -3,8 +3,13 @@
>  
>  __printk_nmi_flush() can be called from nmi_panic(), therefore it has to
>  test whether it's executed in NMI context and thus must route the messages
> -through deferred printk() or via direct printk(). Except for two places
> -where __printk_nmi_flush() does unconditional direct printk() calls:
> +through deferred printk() or via direct printk().  This is to avoid
> +potential deadlocks, as described in cf9b1106c81c45cde ("printk/nmi: flush
> +NMI messages on the system panic").
> +
> +However there remain two places where __printk_nmi_flush() does
> +unconditional direct printk() calls:
> +
>   - pr_err("printk_nmi_flush: internal error ...")
>   - pr_cont("\n")
>  
>