Re: perf: race with automatic rdpmc() disabling
From: Andy Lutomirski
Date: Mon Mar 13 2017 - 17:06:07 EST
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))
>> if (atomic_inc_return(¤t->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?