linux-kernel - Re: [PATCH -v2] tracing: lockdep tracepoints

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Date:	Wed, 04 Mar 2009 14:01:42 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	Török Edwin <edwintorok@...il.com>,
	Jason Baron <jbaron@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -v2] tracing: lockdep tracepoints

On Wed, 2009-03-04 at 13:12 +0100, Frederic Weisbecker wrote:

> The TRACE_FIELD_SPECIAL is only used in case of complex assignment,
> those that can't be done in a simple "=" expression.
> 
> All you need is simply:
> 
> TRACE_FORMAT(lock_contended,
>      TPPROTO(struct lockdep_map *lock, unsigned long ip),
>      TPARGS(lock, ip),
>      TPFMT("%s", lock->name)
>      TRACE_STRUCT(
> 	TRACE_FIELD(char *, name, lock->name)
>      )
>      TPRAWFMT("%s");
> );

As discussed on IRC, that won't actually work. In generic, you cannot
assume the string is stable, in my particular case it would work, since
lock->name is a compile time constant, except for modules, who will
still ruin that.

So I think something like TRACE_FIELD_STRING() is called for. Not sure
how to integrate a variable length field like that with the static
format description though. Maybe use offsets from the start of the
object as string pointers, so you can place the variable sized bits
after the static bits.

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