Re: perf: fuzzer crashes immediately on AMD system

From	Vince Weaver <vincent.weaver@maine.edu>
Newsgroups	linux.kernel
Subject	Re: perf: fuzzer crashes immediately on AMD system
Date	2016-08-23 14:10 +0200
Message-ID	<s9iGJ-3Qa-13@gated-at.bofh.it> (permalink)
References	(2 earlier) <s7OUq-4gK-21@gated-at.bofh.it> <s98xI-5xS-5@gated-at.bofh.it> <s98xI-5xS-3@gated-at.bofh.it> <s9a6u-6rM-3@gated-at.bofh.it> <s9fzb-1v0-11@gated-at.bofh.it>
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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

Thread

Re: perf: fuzzer crashes immediately on AMD system Vince Weaver <vincent.weaver@maine.edu> - 2016-08-18 16:50 +0200
  Re: perf: fuzzer crashes immediately on AMD system Peter Zijlstra <peterz@infradead.org> - 2016-08-19 12:10 +0200
    Re: perf: fuzzer crashes immediately on AMD system Peter Zijlstra <peterz@infradead.org> - 2016-08-19 13:00 +0200
    Re: perf: fuzzer crashes immediately on AMD system Vince Weaver <vincent.weaver@maine.edu> - 2016-08-19 17:10 +0200
      Re: perf: fuzzer crashes immediately on AMD system Vince Weaver <vincent.weaver@maine.edu> - 2016-08-19 18:40 +0200
    Re: perf: fuzzer crashes immediately on AMD system Vince Weaver <vincent.weaver@maine.edu> - 2016-08-20 06:50 +0200
    Re: perf: fuzzer crashes immediately on AMD system Vince Weaver <vincent.weaver@maine.edu> - 2016-08-23 03:20 +0200
      Re: perf: fuzzer crashes immediately on AMD system Vince Weaver <vincent.weaver@maine.edu> - 2016-08-23 05:00 +0200
        Re: perf: fuzzer crashes immediately on AMD system Peter Zijlstra <peterz@infradead.org> - 2016-08-23 10:50 +0200
          Re: perf: fuzzer crashes immediately on AMD system Vince Weaver <vincent.weaver@maine.edu> - 2016-08-23 14:10 +0200
            Re: perf: fuzzer crashes immediately on AMD system Ingo Molnar <mingo@kernel.org> - 2016-08-24 11:30 +0200
              Re: perf: fuzzer crashes immediately on AMD system Vince Weaver <vincent.weaver@maine.edu> - 2016-08-24 15:30 +0200

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