Re: perf: fuzzer crashes immediately on AMD system

[Vince Weaver]

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