linux-kernel - Re: [PATCH v2] kernel/printk: prevent deadlock at unexpected call kmsg

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Date:   Tue, 16 Jul 2019 09:41:04 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] kernel/printk: prevent deadlock at unexpected call
 kmsg_dump in NMI context

On Mon 2019-07-15 11:04:55, Konstantin Khlebnikov wrote:
> Kernel message dumper - function kmsg_dump() is called on various oops or
> panic paths which could happen in unpredictable context including NMI.
> 
> Panic in NMI is handled especially by stopping all other cpus with
> smp_send_stop() and busting locks in printk_safe_flush_on_panic().
> 
> Other less-fatal cases shouldn't happen in NMI and cannot be handled.
> But this might happen for example on oops in nmi context. In this case
> dumper could deadlock on lockbuf_lock or break internal structures.

If I get it correctly than this patch could really prevent a deadlock
in at least:

  + oops_end()
    + oops_exit()
      + kmsg_dump(KMSG_DUMP_OOPS)

If it is called in NMI, it should end up with panic(). Then the dump
will be called later after stopping CPUs...

Or am I wrong?

Otherwise, the patch looks good to me. I would just mention
the above scenario if it is correct.

Best Regards,
Petr

> This patch catches kmsg_dump() called in NMI context except panic and
> prints warning once.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> Link: https://lore.kernel.org/lkml/156294329676.1745.2620297516210526183.stgit@buzz/ (v1)
> ---
>  kernel/printk/printk.c |    7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 1888f6a3b694..e711f64a1843 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3104,6 +3104,13 @@ void kmsg_dump(enum kmsg_dump_reason reason)
>  	struct kmsg_dumper *dumper;
>  	unsigned long flags;
>  
> +	/*
> +	 * In NMI context only panic could be handled safely:
> +	 * it stops other cpus and busts logbuf lock.
> +	 */
> +	if (WARN_ON_ONCE(reason != KMSG_DUMP_PANIC && in_nmi()))
> +		return;
> +
>  	if ((reason > KMSG_DUMP_OOPS) && !always_kmsg_dump)
>  		return;
>  
>