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SubjectRe: perf: fuzzer crashes immediately on AMD system
On Tue, 23 Aug 2016, Peter Zijlstra wrote:

> On Mon, Aug 22, 2016 at 10:54:32PM -0400, Vince Weaver wrote:
> > > > > >
> > > > > > perf stat -a -e amd_nb/config=0x37,config1=0x20/ /bin/ls
> > > amd_uncore_find_online_sibling()
> > > function is broken.
> >
> > and that's the problem. uncore_find_online_sibling() does all kinds of
> > wrong things including sticking active uncore structures in
> > uncore->free_when_cpu_online
> >
> > Then uncore_online() comes along and frees those structures.
> >
> > Then some other part of the kernel comes and re-uses the free'd data.
> >
> > Then when we try to start an event, all of the fields are invalid because
> > the uncore pointer is pointing to re-used data.
> >
> > I don't have a patch because I am not 100% clear on what
> > uncore_find_online_sibling() is doing in the first place.
>
> Thanks for doing all that, I'll see if I can make sense of it.

I should have provided more detail, was just tired after chasing the bug
for so long. I mostly found things by sprinkling printks everywhere.
Comenting out the call to kfree() in uncore_online() makes the code stop
crashing (but perhaps causes a memory leak?)

In any case it's odd the problem didn't show up earlier, but maybe the
recent changes to CPU hotplugging in that file exposed the issue.

Vince

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