Re: perf: race with automatic rdpmc() disabling
From: Peter Zijlstra
Date: Mon Mar 13 2017 - 12:55:25 EST
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().