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Date: Thu, 17 Dec 2015 15:26:17 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Ingo Molnar <mingo@redhat.com>, Arnaldo Carvalho de Melo <acme@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        syzkaller <syzkaller@googlegroups.com>,
        Kostya Serebryany <kcc@google.com>,
        Alexander Potapenko <glider@google.com>,
        Eric Dumazet <edumazet@google.com>,
        Sasha Levin <sasha.levin@oracle.com>
Subject: Re: use-after-free in __perf_install_in_context
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On Thu, Dec 17, 2015 at 03:08:49PM +0100, Dmitry Vyukov wrote:
> On Thu, Dec 17, 2015 at 3:06 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Thu, Dec 10, 2015 at 08:57:40PM +0100, Peter Zijlstra wrote:
> >> I'm not sure I can explain your problem with this, but I figure its
> >> worth a try.
> >
> > Did it make a difference?
> >
> > Also, does the syzkaller thing do cpu-hotplug during its runs?
> 
> See my email on Dec 15.

I have 4 emails from you on that day (and one private), they pertain to
getting stuck in perf_{install,remove}_from_context(). None of those
emails seem to contain an answer to either of the two above questions.

  CACT4Y+Za4L4q6FZLXjyGbnSYRwa7RJZYcG_s562q+CrNy9C75w@mail.gmail.com
  CACT4Y+YUkYgCuQFW6eBTtdfRqdF7EEtrB3VaYJk+LUXE_-iGRQ@mail.gmail.com
  CACT4Y+bvHWuyDZjoFhLAOiK1rfNQcfuSJCY0oS5xA=u8j04bXQ@mail.gmail.com
  CACT4Y+an67KnTsVVzYVvrYKW9yg7PbDuC8WyB38PiWRoSt0UCQ@mail.gmail.com

and in private:

  CACT4Y+Zb1vU192JbCnyS9qY3cpsvfAUJdpBVUsmosueCtQjSXQ@mail.gmail.com

Did I miss an email?

This is the thread about the difficult to deterministically reproduce
use-after-free.

> I have CONFIG_HOTPLUG enabled, but I don't do actual hotplug (well,
> actually I don't know how to do hotplug, but I still think I am not
> doing it).

You can do hotplug like:

echo 0 > /sys/devices/system/cpu/cpu1/online
echo 1 > /sys/devices/system/cpu/cpu1/online

so if syzkaller doesn't know about that, and I suppose you not
knowing this means syzkaller doesn't either, this isn't happening.

Thanks, that disqualifies a bunch of code.