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authorAndy Lutomirski <luto@kernel.org>2017-03-16 12:59:40 -0700
committerIngo Molnar <mingo@kernel.org>2017-03-17 08:28:26 +0100
commit4b07372a32c0c1505a7634ad7e607d83340ef645 (patch)
tree3af5756c3cfe90f9f994356941b5435312168f70 /arch
parent5dc855d44c2ad960a86f593c60461f1ae1566b6d (diff)
downloadlinux-4b07372a32c0c1505a7634ad7e607d83340ef645.tar.bz2
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>
Diffstat (limited to 'arch')
-rw-r--r--arch/x86/events/core.c12
1 files changed, 12 insertions, 0 deletions
diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index e07b36c5588a..183a972f9210 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);
}