linux-kernel - Re: [PATCH 0/2] panic/printk/x86: Prevent some more printk-related deadlocks in panic()

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Message-ID: <CALYGNiPETLPeOOuRwQtYca=yrwWHgg11A6Fc_qZipG56N8KP8A@mail.gmail.com>
Date:   Thu, 18 Jul 2019 12:59:47 +0300
From:   Konstantin Khlebnikov <koct9i@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Ogness <john.ogness@...utronix.de>,
        Petr Tesarik <ptesarik@...e.cz>, x86@...nel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] panic/printk/x86: Prevent some more printk-related
 deadlocks in panic()

On Tue, Jul 16, 2019 at 10:28 AM Petr Mladek <pmladek@...e.com> wrote:
>
> Hi,
>
> I have found some spare duct tape and wrapped some more printk-related
> deadlocks in panic().
>
> More seriously, someone reported a deadlock in panic(). Some non-trivial
> debugging pointed out a problem with the following combination:
>
>      + x86_64 architecture
>      + panic()
>      + pstore configured as message dumper (kmsg_dump())
>      + crash kernel configured
>      + crash_kexec_post_notifiers
>
> In this case, CPUs are stopped by crash_smp_send_stop(). It uses
> NMI but it does not modify cpu_online_mask. Therefore logbuf_lock
> might stay locked, see 2nd patch for more details.
>
> The above is a real corner case. But similar problem seems to be
> even in the common situations on architectures that do not use
> NMI in smp_send_stop() as a fallback, see 1st patch.
>
> Back to the duct tape. I hope that we will get rid of these problems
> with the lockless printk ringbuffer rather sooner than later.
> But it still might take some time. And the two fixes might be
> useful also for stable kernels.

Looks good.

Reviewed-by: Konstantin Khebnikov <khlebnikov@...dex-team.ru>

>
>
> Petr Mladek (2):
>   printk/panic: Access the main printk log in panic() only when safe
>   printk/panic/x86: Allow to access printk log buffer after
>     crash_smp_send_stop()
>
>  arch/x86/kernel/crash.c     |  6 +++++-
>  include/linux/printk.h      |  6 ++++++
>  kernel/panic.c              | 49 +++++++++++++++++++++++++++------------------
>  kernel/printk/printk_safe.c | 37 ++++++++++++++++++++++------------
>  4 files changed, 65 insertions(+), 33 deletions(-)
>
> --
> 2.16.4
>