lkml.org 
[lkml]   [2017]   [Mar]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
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@kernel.org>
AuthorDate: Thu, 16 Mar 2017 12:59:40 -0700
Committer: Ingo Molnar <mingo@kernel.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@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Link: http://lkml.kernel.org/r/03a1e629063899168dfc4707f3bb6e581e21f5c6.1489694270.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@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 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);
}
\
 
 \ /
  Last update: 2017-03-17 11:45    [W:0.186 / U:0.580 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site