linux-kernel - Re: [PATCH 0/2] printk: replace ringbuffer

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Date:   Mon, 17 Feb 2020 12:13:25 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        lijiang <lijiang@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] printk: replace ringbuffer

On 2020-02-14, Petr Mladek <pmladek@...e.com> wrote:
>> I oversaw that devkmsg_open() setup a printk_record and so I did not
>> see to add the extra NULL initialization of text_line_count. There
>> should be be an initializer function/macro to avoid this danger.
>> 
>> John Ogness
>> 
>> The quick fixup:
>> 
>> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
>> index d0d24ee1d1f4..5ad67ff60cd9 100644
>> --- a/kernel/printk/printk.c
>> +++ b/kernel/printk/printk.c
>> @@ -883,6 +883,7 @@ static int devkmsg_open(struct inode *inode, struct file *file)
>>  	user->record.text_buf_size = sizeof(user->text_buf);
>>  	user->record.dict_buf = &user->dict_buf[0];
>>  	user->record.dict_buf_size = sizeof(user->dict_buf);
>> +	user->record.text_line_count = NULL;
>
> The NULL pointer hidden in the structure also complicates the code
> reading. It is less obvious when the same function is called
> only to get the size/count and when real data.

OK.

> I played with it and created extra function to get this information.
>
> In addition, I had problems to follow the code in
> record_print_text_inline(). So I tried to reuse the new function
> and the existing record_printk_text() there.
>
> Please, find below a patch that I ended with. I booted a system
> with this patch. But I guess that I actually did not use the
> record_print_text_inline(). So, it might be buggy.

Yes, there are several bugs. But I see where you want to go with this:

- introduce prb_count_lines() to handle line counting

- introduce prb_read_valid_info() for only reading meta-data and getting
  the line count

- also use prb_count_lines() internally

I will include these changes in v2. I will still introduce the static
inlines to initialize records because readers and writers do it
differently.

Thanks.

John Ogness