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

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Date:	Wed, 4 Mar 2009 08:07:02 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	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, 4 Mar 2009, 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");
> );


Nope, that wont work :-(  If this is saved to userspace, all the user 
would have is some hex value. If they have a copy of the vmlinux image, 
then they could probably translate that value, unless it was a module.

Perhaps something like:

 TRACE_EVENT_FORMAT(lock_contended,
	TPPROTO(struct lockdep_map *lock, unsigned long ip),
	TPARGS(lock, ip),
	TPFMT("%s", lock->name);
	TRACE_STRUCT(
		TRACE_FIELD_SPECIAL(char name[16], name,
			strncpy(name, lock->name, 16));
	),
	TPRAWFMT("%s")
 );

This would require those 16 bytes to be copied with the raw format, but 
then we would always have the name (first 16 bytes anyway) for later use.

-- Steve

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