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

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Date:	Fri, 25 Nov 2011 18:43:31 +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 Thu, Nov 24, 2011 at 08:00:16PM +0100, Johannes Berg wrote:
> On Thu, 2011-11-24 at 16:45 +0100, Frederic Weisbecker wrote:
> 
> > > I don't really feel comfortable modifying the _call_console_drivers()
> > > function to not handle start > end (modulo log buf size of course), but
> > > at the same time I don't feel comfortable putting code into it that
> > > doesn't handle it.
> > 
> > So, the:
> > 
> > 	BUG_ON(((int)(start - end)) > 0);
> > 
> > check is there to ensure we haven't wrapped INT_MAX. If we have reached
> > that point it definetly means we have a bug because log_buf_len is itself
> > an int and we shouldn't overlap INT_MAX.
> 
> Ok that makes sense.
> 
> > The care on the wrapping that is done in _call_console_drivers() is
> > different and concerns log_buf_len itself. If log_buf_len = 8, start = 7
> > and end = 9, then you will enter the "((start & LOG_BUF_MASK) > (end & LOG_BUF_MASK))"
> > condition that handle the wrap on LOG_BUF_MASK to print the two chars.
> > But this is totally different from "start > end" which would mean we have
> > a bug.
> 
> Oh. So we get end = 9 in that case? That seems confusing ... I would
> have expected end = 1 then! Which is the whole reason I got confused I
> guess.

end = 9 but (end & LOG_BUF_MASK) = 1

> 
> > So, in your tracepoint you can safely use "end - start" as a length for your
> > dynamic array. But the rest of your tracepoint (all the fast assign part)
> > still needs the masks as you did.
> 
> Oh, that's all you were trying to say?

Yeah I first thought my comment applied to all the mask game in the tracepoint but then
I started to feel ashamed when I realized it was only on the dynamic array declaration :)

> I can see that, ok. I just didn't
> see that end would be 9 instead of 1 and tried to handle that. The
> _call_console_drivers() code is a bit different I guess.
> 
> I'll send a new version.

Thanks.

> 
> johannes
> 
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