linux-kernel - Re: [PATCH printk 1/5] printk: get new seq before enabling interrupts

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Message-ID: <87r1qszhjs.fsf@jogness.linutronix.de>
Date:   Wed, 23 Sep 2020 16:42:23 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk 1/5] printk: get new seq before enabling interrupts

On 2020-09-23, Petr Mladek <pmladek@...e.com> wrote:
>> After copying all records to the dynamic ringbuffer, setup_log_buf()
>> checks to see if any records were dropped during the switch. However,
>> it needs to check before enabling interrupts since new records could
>> arrive in an interrupt, thus causing setup_log_buf() to erroneously
>> think that it had dropped messages.
>
> Have you seen the problem, please?
>
>> Signed-off-by: John Ogness <john.ogness@...utronix.de>
>> ---
>>  kernel/printk/printk.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>> 
>> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
>> index 1fe3d0cb2fe0..00bc1fce3299 100644
>> --- a/kernel/printk/printk.c
>> +++ b/kernel/printk/printk.c
>> @@ -1181,12 +1181,12 @@ void __init setup_log_buf(int early)
>>  	 */
>>  	prb = &printk_rb_dynamic;
>>  
>> +	seq = prb_next_seq(&printk_rb_static) - seq;
>> +
>>  	logbuf_unlock_irqrestore(flags);
>>  
>> -	if (seq != prb_next_seq(&printk_rb_static)) {
>
> I can't see how these two values could get modified after enabling interrupts.
>
>   + @seq is set in the for-cycle before the interrupts are enabled.
>
>   + @prb is updated before the interrupts are  enabled. So that
>     the static buffer should not longer be used after that.

You are correct. This patch should be disregarded.

John Ogness