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

From: Nick Piggin
Date: Mon Aug 02 2010 - 07:17:45 EST

On Mon, Aug 02, 2010 at 11:17:14AM +0200, Ingo Molnar wrote:
> * Nick Piggin <npiggin@xxxxxxx> wrote:
> > On Mon, Aug 02, 2010 at 09:54:22AM +0200, Ingo Molnar wrote:
> > >
> > > * Nick Piggin <npiggin@xxxxxxx> 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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