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From:   Marco Elver <elver@google.com>
Date:   Tue, 8 Feb 2022 14:38:22 +0100
Message-ID: <CANpmjNOySkeK6u-JieNBQ4DmAO3LogdZ6gXv1Noz8jUOi3ThDA@mail.gmail.com>
Subject: Re: [PATCH v2 3/5] panic: unset panic_on_warn inside panic()
To:     Tiezhu Yang <yangtiezhu@loongson.cn>
Cc:     Baoquan He <bhe@redhat.com>, Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        Xuefeng Li <lixuefeng@loongson.cn>, kexec@lists.infradead.org,
        linux-doc@vger.kernel.org, kasan-dev@googlegroups.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
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On Tue, 8 Feb 2022 at 13:51, Tiezhu Yang <yangtiezhu@loongson.cn> wrote:
>
> In the current code, the following three places need to unset
> panic_on_warn before calling panic() to avoid recursive panics:
>
> kernel/kcsan/report.c: print_report()
> kernel/sched/core.c: __schedule_bug()
> mm/kfence/report.c: kfence_report_error()
>
> In order to avoid copy-pasting "panic_on_warn = 0" all over the
> places, it is better to move it inside panic() and then remove
> it from the other places.
>
> Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>

Reviewed-by: Marco Elver <elver@google.com>


> ---
>  kernel/panic.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/panic.c b/kernel/panic.c
> index 55b50e0..95ba825 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -185,6 +185,16 @@ void panic(const char *fmt, ...)
>         int old_cpu, this_cpu;
>         bool _crash_kexec_post_notifiers = crash_kexec_post_notifiers;
>
> +       if (panic_on_warn) {
> +               /*
> +                * This thread may hit another WARN() in the panic path.

Alas, this may actually fix another problem: doing a panic() not from
a WARN(), but then hitting a WARN() along in the panic path. So
"another WARN" is irrelevant, just "a WARN" would be enough to break
things.

> +                * Resetting this prevents additional WARN() from panicking the
> +                * system on this thread.  Other threads are blocked by the
> +                * panic_mutex in panic().
> +                */
> +               panic_on_warn = 0;
> +       }
> +
>         /*
>          * Disable local interrupts. This will prevent panic_smp_self_stop
>          * from deadlocking the first cpu that invokes the panic, since
> @@ -576,16 +586,8 @@ void __warn(const char *file, int line, void *caller, unsigned taint,
>         if (regs)
>                 show_regs(regs);
>
> -       if (panic_on_warn) {
> -               /*
> -                * This thread may hit another WARN() in the panic path.
> -                * Resetting this prevents additional WARN() from panicking the
> -                * system on this thread.  Other threads are blocked by the
> -                * panic_mutex in panic().
> -                */
> -               panic_on_warn = 0;
> +       if (panic_on_warn)
>                 panic("panic_on_warn set ...\n");
> -       }
>
>         if (!regs)
>                 dump_stack();
> --
> 2.1.0
>
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