From: Andy Lutomirski <luto@kernel.org> To: X86 ML <x86@kernel.org> Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Borislav Petkov <bpetkov@suse.de>, Vince Weaver <vincent.weaver@maine.edu>, Andy Lutomirski <luto@kernel.org> Subject: [PATCH 2/2] x86/perf: Clarify why x86_pmu_event_mapped() isn't racy Date: Thu, 16 Mar 2017 12:59:40 -0700 Message-ID: <03a1e629063899168dfc4707f3bb6e581e21f5c6.1489694270.git.luto@kernel.org> (raw) In-Reply-To: <cover.1489694270.git.luto@kernel.org> In-Reply-To: <cover.1489694270.git.luto@kernel.org> Naively, it looks racy, but mmap_sem saves it. Add a comment and a lockdep assertion. Signed-off-by: Andy Lutomirski <luto@kernel.org> --- arch/x86/events/core.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c index 4f564df73b8f..2aa1ad194db2 100644 --- a/arch/x86/events/core.c +++ b/arch/x86/events/core.c @@ -2110,6 +2110,18 @@ static void x86_pmu_event_mapped(struct perf_event *event) if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED)) return; + /* + * This function relies on not being called concurrently in two + * tasks in the same mm. Otherwise one task could observe + * perf_rdpmc_allowed > 1 and return all the way back to + * userspace with CR4.PCE clear while another task is still + * doing on_each_cpu_mask() to propagate CR4.PCE. + * + * For now, this can't happen because all callers hold mmap_sem + * for write. If this changes, we'll need a different solution. + */ + lockdep_assert_held_exclusive(¤t->mm->mmap_sem); + if (atomic_inc_return(¤t->mm->context.perf_rdpmc_allowed) == 1) on_each_cpu_mask(mm_cpumask(current->mm), refresh_pce, NULL, 1); } -- 2.9.3
next prev parent reply index Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2017-03-16 19:59 [PATCH 0/2] x86/perf: A CR4.PCE bugfix and clarification Andy Lutomirski 2017-03-16 19:59 ` [PATCH 1/2] x86/perf: Fix CR4.PCE propagation to use active_mm instead of mm Andy Lutomirski 2017-03-17 10:16 ` [tip:perf/urgent] " tip-bot for Andy Lutomirski 2017-03-16 19:59 ` Andy Lutomirski [this message] 2017-03-17 10:17 ` [tip:perf/urgent] x86/perf: Clarify why x86_pmu_event_mapped() isn't racy tip-bot for Andy Lutomirski
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