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

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Message-ID: <5a101004-72b5-3699-b39d-e1acac996551@yandex-team.ru>
Date:   Tue, 16 Jul 2019 10:55:14 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Petr Mladek <pmladek@...e.com>
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 16.07.2019 10:41, Petr Mladek wrote:
> 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?

Yep. Under 'oops in nmi context' I mean exactly that case.

> 
> 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;
>>   
>>