linux-kernel - Re: perf: perf_fuzzer lockup in perf_cgroup

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Date:   Thu, 15 Sep 2016 08:41:52 -0400 (EDT)
From:   Vince Weaver <vincent.weaver@...ne.edu>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     linux-kernel@...r.kernel.org,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Stephane Eranian <eranian@...gle.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: perf: perf_fuzzer lockup in perf_cgroup_attach

On Thu, 15 Sep 2016, Peter Zijlstra wrote:

> On Wed, Sep 14, 2016 at 10:43:29PM -0400, Vince Weaver wrote:
> > 
> > so the skylake that was fuzzing finally is mostly locked up.
> > 
> > Really hard to tell what's going, especially as KASLR made looking up the 
> > addresses a big pain.
> > 
> > The best I can tell things are getting wedged somehow in 
> > perf_cgroup_switch() while interrupts are disabled.  Interrupts are never 
> > getting re-enabled, causing the RCU and NMI watchdogs to trigger (and more 
> > alarming things like the SATA bus resetting).
> 
> How do you go about using cgroups? Do you set them up yourself, does the
> fuzzer do so?

that's an interesting thing, I don't think the fuzzer actually sets up any 
cgroups.  It will try creating events with PERF_FLAG_PID_CGROUP but it 
will just use a random number as the cgroup fd.  

I know at one time I meant to do more interesting things with cgroups
but if I recall it wasn't really possible without running as root and I 
usually don't fuzz as root.

Vince