linux-kernel - Re: [PATCH] tracing/lockdep: turn lock->name into an array

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Message-ID: <20090414065303.GA24626@elte.hu>
Date:	Tue, 14 Apr 2009 08:53:03 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Zhaolei <zhaolei@...fujitsu.com>,
	Tom Zanussi <tzanussi@...il.com>,
	Li Zefan <lizf@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] tracing/lockdep: turn lock->name into an array


* KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:

> Hi
> 
> > Impact: allow filtering by lock name / fix module tracing
> > 
> > Currently, the "lock acquired" event is traced using a TRACE_EVENT.
> > But we can't use the char * type for the name without risking to
> > dereference a freed pointer. A lock name can come from a module
> > towards lockdep and it is risky to only store its address because we
> > defer its name printing.
> 
> When released lockdep string table? I guess it only happend at 
> module unloading. if so, we should consider to make delayed string 
> table freeing at module unloading.
> 
> My point is, module unloading is rare event. thus meking pointer 
> safe mechanism widely avoid string copy.
> 
> IOW, if not, ringbuffer is filled tons string. it kill the merit 
> of binary buffer and current design.

We could zap all pending trace entries on module unload (it is a 
rare operation). That would indeed make a whole category of 
symbol-alike string pointers safe to be passed by value.

	Ingo
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