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Date: Tue, 14 Apr 2009 12:25:33 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Peter Zijlstra <peterz@infradead.org>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Frederic Weisbecker <fweisbec@gmail.com>,
       Steven Rostedt <rostedt@goodmis.org>, Zhaolei <zhaolei@cn.fujitsu.com>,
       Tom Zanussi <tzanussi@gmail.com>, Li Zefan <lizf@cn.fujitsu.com>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] tracing/lockdep: turn lock->name into an array
Message-ID: <20090414102533.GG3558@elte.hu>
References: <1239662166-13208-1-git-send-email-fweisbec@gmail.com> <20090414153035.C662.A69D9226@jp.fujitsu.com> <20090414065303.GA24626@elte.hu> <1239693611.21985.6694.camel@twins>
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* Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, 2009-04-14 at 08:53 +0200, Ingo Molnar wrote:
> > * KOSAKI Motohiro <kosaki.motohiro@jp.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.
> 
> Except that might make debugging a module unload bug rather 
> hard...

yes. Never needed to do that personally though. Plus more specific 
tracers like function tracer without event-tracing embellishments 
could still be used in such situations too, without flushing.

	Ingo