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

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Date:	Fri, 18 Nov 2011 19:54:56 +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 07:46:15PM +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)
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