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

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Date:	Wed, 6 May 2009 19:13:26 +0200
From:	Christoph Hellwig <hch@....de>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Christoph Hellwig <hch@....de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>, cbe-oss-dev@...abs.org,
	srostedt@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH, RFC] sputrace: use the generic event tracer

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.

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