linux-kernel - Re: [PATCH 1/2] mmiotrace: fix buffer overrun detection

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Message-ID: <20081124232720.59312c0d@daedalus.pq.iki.fi>
Date:	Mon, 24 Nov 2008 23:27:20 +0200
From:	Pekka Paalanen <pq@....fi>
To:	"Frédéric Weisbecker" <fweisbec@...il.com>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Steven Rostedt" <rostedt@...dmis.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mmiotrace: fix buffer overrun detection

On Mon, 24 Nov 2008 21:30:24 +0100
"Frédéric Weisbecker" <fweisbec@...il.com> wrote:

> 2008/11/24 Pekka Paalanen <pq@....fi>:
> >
> > Oh right, the busylooping bug when echoing into trace_marker,
> > I already forgot about that, since I couldn't reproduce it.
> > I guess there's nothing to do, until someone finds a way to
> > reproduce it.
> 
> 
> Hi Pekka.
> 
> Are you talking about this old bug that broke the pipe?
> If so, you fixed it yourself partially with a patch that corrected
> tracing_read_pipe.
> Another origin was the none tracer that set tracer_enabled to 0.
> But i has been fixed with the nop tracer.
> 
> Perhaps you are talking of something else?

Yeah, I'm talking about the shell spinning 90% in kernel and 10% in
user, I tried to explain it in the part you left out from the quote.
That is, doing 'echo foo > trace_marker' makes the shell/echo spin.
And this doesn't seem to be deterministic, or the exact triggering
sequence is too complex. I didn't notice any other adverse effects
than the shell getting stuck.

Maybe a corner case related to having only a single page in the
ring buffer... nah, more likely something unexpected under
trace_marker callbacks. Returning zero instead of error on
write(2) maybe... something that makes 'echo' retry again and
again, never succeeding or really failing.

-- 
Pekka Paalanen
http://www.iki.fi/pq/
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