linux-kernel - Re: [RFC PATCH v1 11/25] printk

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Date:   Fri, 22 Feb 2019 14:38:28 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH v1 11/25] printk_safe: remove printk safe code

On 2019-02-22, Petr Mladek <pmladek@...e.com> wrote:
>> diff --git a/lib/nmi_backtrace.c b/lib/nmi_backtrace.c
>> index 15ca78e1c7d4..77bf84987cda 100644
>> --- a/lib/nmi_backtrace.c
>> +++ b/lib/nmi_backtrace.c
>> @@ -75,12 +75,6 @@ void nmi_trigger_cpumask_backtrace(const cpumask_t *mask,
>>  		touch_softlockup_watchdog();
>>  	}
>>  
>> -	/*
>> -	 * Force flush any remote buffers that might be stuck in IRQ context
>> -	 * and therefore could not run their irq_work.
>> -	 */
>> -	printk_safe_flush();
>> -
>>  	clear_bit_unlock(0, &backtrace_flag);
>>  	put_cpu();
>>  }
>
> This reminds me that we need to add back the locking that was
> removed in the commit 03fc7f9c99c1e7ae2925d45 ("printk/nmi:
> Prevent deadlock when accessing the main log buffer in NMI").

No, that commit is needed. You cannot have NMIs waiting on other CPUs.

> Otherwise, backtraces from different CPUs would get mixed.

A later patch (#17) adds CPU IDs to the printk messages so that this
isn't a problem. (That patch is actually obsolete now because Sergey has
already merged work for linux-next that includes this information.)

John Ogness