linux-kernel - Re: [PATCH, RFC] sputrace: use the generic event tracer

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Message-Id: <1241636739.11379.29.camel@localhost.localdomain>
Date:	Wed, 06 May 2009 15:05:39 -0400
From:	Steven Rostedt <srostedt@...hat.com>
To:	Christoph Hellwig <hch@....de>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>, cbe-oss-dev@...abs.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH, RFC] sputrace: use the generic event tracer


On Wed, 2009-05-06 at 19:13 +0200, Christoph Hellwig wrote:
> On Wed, May 06, 2009 at 06:53:37PM +0200, Frederic Weisbecker wrote:
> > I don't know much the code you are tracing. But it is rare that
> > a const char * is safe on tracing. Still it could be, you just have to
> > ensure the string cannot be freed in any way because this pointer
> > will be stored in the ring buffer and it can be read and dereferenced later
> > in a random time, could be several years :-)
> > 
> > So if this pointer references built-in data, no problem with that.
> > But if it can freed (comes from a module, __initdata, ...), then
> > you should use the __string() field which does an strcpy on the
> > ring buffer.
> > 
> > If you think this is safe, then it's the best choice because
> > storing a pointer is of course less costly than an strcpy.
> > If so I will add the support for char * in the filters (trivial).
> 
> The pointer here only ever references string constants, it's always
> a string literal in the callers.

The worry is if this is used by modules. A constant string may not be
around when the buffer is read.

This should not be a problem because the formatting of the string is not
around either, and we just output 'unknown type'.  But I may be adding
code when a module is unloaded to reset the ring buffer if the module
registered any events. That's because we have other races to worry
about.

-- Steve


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