linux-kernel - Re: [PATCH 11/19] perf record: Release resources at exit

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Date:	Mon, 2 Aug 2010 11:17:14 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Arnaldo Carvalho de Melo <acme@...radead.org>,
	linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 11/19] perf record: Release resources at exit


* Nick Piggin <npiggin@...e.de> wrote:

> On Mon, Aug 02, 2010 at 09:54:22AM +0200, Ingo Molnar wrote:
> > 
> > * Nick Piggin <npiggin@...e.de> wrote:
> > 
> > > On Sun, Aug 01, 2010 at 10:08:46PM -0300, Arnaldo Carvalho de Melo wrote:
> > > > From: Arnaldo Carvalho de Melo <acme@...hat.com>
> > > > 
> > > > So that we can reduce the noise on valgrind when looking for memory
> > > > leaks.
> > > 
> > > Really? That's rather crappy of valgrind. exit is well defined to release 
> > > resources and that's often a more efficient way to do it It finds and 
> > > batches things a lot better, eg. it can avoid all TLB flushing of freeing 
> > > memory that munmap requires.
> > 
> > That's certainly true but there's no valgrind crappiness here: valgrind simply 
> > can do a better job of finding leaks if there's a well defined "all resources 
> > the app still knows about are freed now" point.
> 
> "noise" sounds like false positives though. [...]

Every predictive bug detection scheme is open to the potential of false 
positives. I've yet to see a complex one that is 100% false positive free.

> [...] Certainly if this is instead allows valgrind to run in a particular 
> mode that assumes no application resources consumed at exit(2) time, I 
> wouldn't call it crappy :)

Most apps free their stuff before they exit - i do it in all my own C apps.

That is generally useful: for example it makes it easier to thread a program 
later on - when exit() becomes pthread_exit() and a silent leak turns into a 
real leak.

Hence Valgrind checking for exit() by default looks useful to me.

> But you could equally sprinkle in other valgrind specific annotations or 
> semantics at various points in the code to improve its coverage, no?

Yeah, and exit() sounds like a pretty convenient point, right? That's the 
point where all resources are inactive hence a scan for leaks is expected to 
be the most efficient in finding real leaks.

Thanks,

	Ingo
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