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

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Date:	Mon, 2 Aug 2010 19:59:47 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Nick Piggin <npiggin@...e.de>,
	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

On Mon, Aug 02, 2010 at 11:17:14AM +0200, Ingo Molnar wrote:
> 
> * 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:
> > > 
> > > 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.

So long as false positive noise can be easily avoided in running
of the automated testing -- ie. not worked around in the software --
then this is totally fine of course.

 
> > [...] 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.

Sure, most of the time I would too (in fact, all the time seeing as
I don't write non-trivial user programs). But when you're doing last
pass optimisations, it would be very reasonable to avoid things like
teardown of complex data structures.

 
> > 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.

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