linux-kernel - [PATCH 2/2] x86/perf: Clarify why x86_pmu_event

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Message-Id: <03a1e629063899168dfc4707f3bb6e581e21f5c6.1489694270.git.luto@kernel.org>
Date:   Thu, 16 Mar 2017 12:59:40 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     X86 ML <x86@...nel.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bpetkov@...e.de>,
        Vince Weaver <vincent.weaver@...ne.edu>,
        Andy Lutomirski <luto@...nel.org>
Subject: [PATCH 2/2] x86/perf: Clarify why x86_pmu_event_mapped() isn't racy

Naively, it looks racy, but mmap_sem saves it.  Add a comment and a
lockdep assertion.

Signed-off-by: Andy Lutomirski <luto@...nel.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(&current->mm->mmap_sem);
+
 	if (atomic_inc_return(&current->mm->context.perf_rdpmc_allowed) == 1)
 		on_each_cpu_mask(mm_cpumask(current->mm), refresh_pce, NULL, 1);
 }
-- 
2.9.3