Re: perf: race with automatic rdpmc() disabling

[Andy Lutomirski]

On Tue, Mar 14, 2017 at 9:45 AM, Vince Weaver <vincent.weaver@xxxxxxxxx> wrote:
> On Tue, 14 Mar 2017, Andy Lutomirski wrote:
>
>> On Mon, Mar 13, 2017 at 2:05 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> > On Mon, Mar 13, 2017 at 9:55 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>> >> On Mon, Mar 13, 2017 at 09:44:02AM -0700, Andy Lutomirski wrote:
>> >>> static void x86_pmu_event_mapped(struct perf_event *event)
>> >>> {
>> >>>     if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED))
>> >>>         return;
>> >>>
>> >>>     if (atomic_inc_return(&current->mm->context.perf_rdpmc_allowed) == 1)
>> >>>
>> >>> <-- thread 1 stalls here
>> >>>
>> >>>         on_each_cpu_mask(mm_cpumask(current->mm), refresh_pce, NULL, 1);
>> >>> }
>> >>>
>> >>> Suppose you start with perf_rdpmc_allowed == 0.  Thread 1 runs
>> >>> x86_pmu_event_mapped and gets preempted (or just runs slowly) where I
>> >>> marked.  Then thread 2 runs the whole function, does *not* update CR4,
>> >>> returns to userspace, and GPFs.
>> >>>
>> >>> The big hammer solution is to stick a per-mm mutex around it.  Let me
>> >>> ponder whether a smaller hammer is available.
>> >>
>> >> Reminds me a bit of what we ended up with in kernel/jump_label.c:static_key_slow_inc().
>> >>
>> >>
>> >
>> > One thing I don't get: isn't mmap_sem held for write the whole time?
>>
>> mmap_sem is indeed held, so my theory is wrong.  I can reproduce it,
>> but I don't see the bug yet...
>
> It could still be a PAPI bug, as I'm having absolutely no luck trying to
> come up with a plain perf_event reproducer.
>
> Let me dig through the PAPI code again and make sure I'm not missing
> something.

Can you give this a try:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/fixes&id=9edb8154863ba1a7f6f1f15ffe6aecf3cf32bf21

(The link doesn't work yet but it should in a minute or two.)

--Andy