Re: [Valgrind-users] DRD and --free-is-write bug-report

[Konstantin S. <kon...@gm...>]

FYI
ThreadSanitizer will not report anything on this test, even with
--free-is-write (which is on by default) because this tool assumes the
malloc implementation to be correct and ignores every access inside malloc
and friends.
I haven't seen any false positives due to --free-is-write (in
ThreadSanitizer) over last ~6 months, but we found tons of real races with
this flag. Just a fresh example:
http://code.google.com/p/chromium/issues/detail?id=72042

--kcc


On Fri, Feb 4, 2011 at 10:16 PM, Bart Van Assche <bva...@ac...> wrote:

> On Fri, Feb 4, 2011 at 12:16 PM, Andrea Mazzoleni <ama...@gm...>
> wrote:
> >
> > I likely found a problem in the DRD when using the --free-is-write
> > option. The following test case reports an huge amount of errors that
> > are not expected.
> >
> > Valgrind and helgrind report no error.
> >
> > The machine is a Ubuntu 10.4 x86 with valgrind 3.6.0 compiled from
> source.
> >
> > The program is compiled with: gcc -O1 -g -D_REENTRANT  multi.c -o
> > multi -lpthread
> >
> > In a big application I have similar strange reports, even without
> > using --free-is-write, so maybe --free-is-write is only a way to
> > expose the bug.
>
> The implementation of --free-is-write in DRD is based on the
> assumption that all modifications of memory after a call to free() and
> before a new malloc() call returns it again are errors. I'm afraid
> this assumption is wrong (the memory allocator may touch such memory)
> so I have removed the --free-is-write option again.
>
> Regarding the behavior of DRD with --free-is-write=no: DRD in this
> configuration is working reliably for many people since considerable
> time. So the issues reported with this configuration are most likely
> either application errors or false positives triggered by the
> distro-specific libthread or libc implementation. One should keep in
> mind that the data race detection algorithms in DRD and Helgrind are
> different and hence that both tools may report different data races or
> false positives.
>
> Further feedback for the behavior of DRD with --free-is-write=no is
> welcome.
>
> Bart.
>
>
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