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

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Date:   Mon, 17 Feb 2020 15:50:16 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
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 Mon 2020-02-17 12:13:25, John Ogness wrote:
> 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

In addition, I would like share the code between
record_print_text_inline() and record_print_text().

They both do very similar thing and the logic in far from
trivial.

Alternative solution would be to get rid of record_print_text()
and use record_print_text_inline() everywhere. It will have some
advantages:

  + _inline() variant will get real testing
  + no code duplication
  + saving the extra buffer also in console, sysfs, and devkmsg interface.


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

Sounds good.

Best Regards,
Petr