linux-kernel - Re: [PATCH v2] printk: add console output tracing

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Message-ID: <20111123131622.GA10669@somewhere.redhat.com>
Date:	Wed, 23 Nov 2011 14:16:28 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Christoph Hellwig <hch@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2] printk: add console output tracing

On Fri, Nov 18, 2011 at 07:59:12PM +0100, Johannes Berg wrote:
> On Fri, 2011-11-18 at 19:54 +0100, Frederic Weisbecker wrote:
> > On Fri, Nov 18, 2011 at 07:46:15PM +0100, Johannes Berg wrote:
> > > On Fri, 2011-11-18 at 19:44 +0100, Frederic Weisbecker wrote:
> > > 
> > > > > +TRACE_EVENT_CONDITION(console,
> > > > > +	TP_PROTO(const char *log_buf, unsigned start, unsigned end,
> > > > > +		 unsigned log_buf_len),
> > > > > +
> > > > > +	TP_ARGS(log_buf, start, end, log_buf_len),
> > > > > +
> > > > > +	TP_CONDITION(start != end),
> > > > > +
> > > > > +	TP_STRUCT__entry(
> > > > > +		__dynamic_array(char, msg,
> > > > > +				((end - start + log_buf_len) &
> > > > > +				 (log_buf_len - 1)) + 1)
> > > > 
> > > > Is all that care about log_buf_len necessary? It seems that
> > > > printk ensures that log_end - con_start never exceeds log_buf_len,
> > > > looking at emit_log_char()
> > > 
> > > I think it is. The buffer can wrap around so in that case end < start,
> > > which just end-start won't handle here.
> > 
> > Even if it wraps, end - start should always give a positive result.
> > 
> > We have that check in call_console_drivers():
> > 
> > 	BUG_ON(((int)(start - end)) > 0)
> 
> That's .. confusing, start - end > 0 <=> start > end ??

Yeah. Both are unsigned, so converting the result into an int
and getting a positive value means we have wrapped INT_MAX.

> 
> Also, call_console_drivers() doesn't pass this start/end through to
> _call_console_drivers(), it has loops and stuff there...

Yeah but looking at the loop there, start and end passed to
__call_console_drivers() are bounded between start and end passed to
call_console_drivers().

> 
> In any case -- feel free to clean it all up, I basically copied the
> logic from _call_console_drivers assuming it was needed.

I'm just reviewing and suggesting a way to keep the patch more simple.
Outside that, I don't really need that patch myself ;)

> I don't really want to know about the printk details :-)

Well, it's like working out outside in winter. It may not sound
very entertaining in the beginning but then it quickly becomes
invigorating, toning and good for the mood...I think...
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