linux-kernel - Re: [PATCH] kernel/printk: prevent deadlock at calling kmsg

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Date:   Sat, 13 Jul 2019 09:46:56 +0300
From:   Konstantin Khlebnikov <koct9i@...il.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Petr Mladek <pmladek@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] kernel/printk: prevent deadlock at calling kmsg_dump from
 NMI context

On Sat, Jul 13, 2019 at 9:10 AM Sergey Senozhatsky
<sergey.senozhatsky@...il.com> wrote:
>
> On (07/12/19 17:54), Konstantin Khlebnikov wrote:
> > Function kmsg_dump could be invoked from NMI context intentionally or
> > accidentally because it is called at various oops/panic paths.
> > Kernel message dumpers are not ready to work in NMI context right now.
> > They could deadlock on lockbuf_lock or break internal structures.
>
> Hmm.
> printk()-s from NMI go through per-CPU printk_safe/nmi - a bunch of
> lockless buffers which is supposed to deal with printk() deadlocks,
> including NMI printk()-s.
>
> include/linux/hardirq.h
>
> #define nmi_enter()
>         ...
>         printk_nmi_enter();
>         ...
>
> #define nmi_exit()
>         ...
>         printk_nmi_exit();
>         ...
>
> So we are not really supposed to deadlock.

Yep printk() can deal with NMI, but kmsg_dump() is a different beast.
It reads printk buffer and saves content into persistent storage like ACPI ERST.

>
>         -ss