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>

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index e07b36c5588afa95c42a880ea56898b0fd27e83e..183a972f9210fb4f7a7dd8b5223c58f1bfac018c 100644 (file)
--- 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);
 }