Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 14 Mar 2017 08:58:36 -0700 | Subject | Re: perf: race with automatic rdpmc() disabling |
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On Mon, Mar 13, 2017 at 2:05 PM, Andy Lutomirski <luto@kernel.org> wrote: > On Mon, Mar 13, 2017 at 9:55 AM, Peter Zijlstra <peterz@infradead.org> 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(¤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?
mmap_sem is indeed held, so my theory is wrong. I can reproduce it, but I don't see the bug yet...
--Andy
-- Andy Lutomirski AMA Capital Management, LLC
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