linux-kernel - [tip:perf/urgent] x86/perf: Clarify why x86_pmu_event

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Date:   Fri, 17 Mar 2017 03:17:17 -0700
From:   tip-bot for Andy Lutomirski <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     torvalds@...ux-foundation.org, alexander.shishkin@...ux.intel.com,
        linux-kernel@...r.kernel.org, bpetkov@...e.de, luto@...nel.org,
        tglx@...utronix.de, vincent.weaver@...ne.edu, eranian@...gle.com,
        jolsa@...hat.com, hpa@...or.com, mingo@...nel.org,
        peterz@...radead.org, acme@...hat.com
Subject: [tip:perf/urgent] x86/perf: Clarify why x86_pmu_event_mapped()
 isn't racy

Commit-ID:  4b07372a32c0c1505a7634ad7e607d83340ef645
Gitweb:     http://git.kernel.org/tip/4b07372a32c0c1505a7634ad7e607d83340ef645
Author:     Andy Lutomirski <luto@...nel.org>
AuthorDate: Thu, 16 Mar 2017 12:59:40 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 17 Mar 2017 08:28:26 +0100

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>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
Cc: Borislav Petkov <bpetkov@...e.de>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Jiri Olsa <jolsa@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Stephane Eranian <eranian@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Vince Weaver <vincent.weaver@...ne.edu>
Link: http://lkml.kernel.org/r/03a1e629063899168dfc4707f3bb6e581e21f5c6.1489694270.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@...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 e07b36c..183a972 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -2109,6 +2109,18 @@ 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);
 }