linux-kernel - Re: vprintk_store: was: [PATCH next v2 3/3] printk: remove logbuf_lock, add syslog

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Message-ID: <X84kEEGwMGGELlWt@alley>
Date:   Mon, 7 Dec 2020 13:46:08 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
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: vprintk_store: was: [PATCH next v2 3/3] printk: remove
 logbuf_lock, add syslog_lock

On Sun 2020-12-06 23:36:53, John Ogness wrote:
> On 2020-12-04, Petr Mladek <pmladek@...e.com> wrote:
> >> +	if (facility == 0) {
> >> +		while (text_len >= 2 && printk_get_level(text)) {
> >> +			text_len -= 2;
> >> +			text += 2;
> >> +		}
> >
> > We should avoid two completely different approaches
> > that handle printk_level prefix.
> >
> > One solution is to implement something like:
> >
> >      static char *parse_prefix(text, &level, &flags)
> >
> > That would return pointer to the text after the prefix.
> > And fill level and flags only when non-NULL pointers are passed.
> 
> OK.
> 
> > Another solution would be to pass this information from
> > vprintk_store(). The prefix has already been parsed
> > after all.
> 
> Well, there is a vscnprintf() that happens in between and I don't think
> we should trust the parsed offset from the first vsnprintf().

Good point!

> >> +
> >> +		if (text != orig_text)
> >> +			memmove(orig_text, text, text_len);
> >> +	}
> >
> > We should clear the freed space to make the ring buffer as
> > human readable as possible when someone just dumps the memory.
> 
> Data blocks are currently padded and that padding is not cleared. So it
> is already not perfectly human readable on a raw dump.

It would be nice to clean up the padding as well. But it is a cosmetic
improvement that might be done anytime later.


> > Sigh, I have to admit that I missed the problem with prefix and
> > trailing '\n' when I suggested to avoid the temporary buffers.
> > This memmove() and the space wasting is pity.
> >
> > Well, it is typically 3 bytes per message. And the copying would
> > be necessary even with the temporary buffer. So, I am less convinced
> > but I would still try to avoid the temporary buffers for now.
> 
> Agreed. I think this approach is better than the temporary buffers I
> previously used.

Another motivation is that it allows to simply handle recursion/nesting.
Othrewise, we would need temporary buffers for each allowed recursion
level or some tricky code.

> Also, if we add a trimming feature to the ringbuffer,
> it will keep the ringbuffer mostly clean anyway. Something like this:
> 
> prb_rec_init_wr(&r, text_len);
> prb_reserve(&e, prb, &r);
> text_len = printk_sprint(&r.text_buf[0], text_len, ...);
> r.info->text_len = text_len;
> prb_trim_rec(&e, &r); <--- try to reduce datablock size to @text_len
> prb_commit(&e);
> 
> I see no urgency to add such a feature. But I think we should keep it on
> our radar.

Yup. I thought about it as well. I agree that it is not a priority.

Best Regards,
Petr