3.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Peter Zijlstra <pet...@infradead.org>


[ Upstream commit ba5213ae6b88fb170c4771fef6553f759c7d8cdd ]

Andi was asking about PERF_FORMAT_GROUP vs inherited events, which led
to the discovery of a bug from commit:

  3dab77fb1bf8 ("perf: Rework/fix the whole read vs group stuff")

 -       PERF_SAMPLE_GROUP                       = 1U << 4,
 +       PERF_SAMPLE_READ                        = 1U << 4,

 -       if (attr->inherit && (attr->sample_type & PERF_SAMPLE_GROUP))
 +       if (attr->inherit && (attr->read_format & PERF_FORMAT_GROUP))

is a clear fail :/

While this changes user visible behaviour; it was previously possible
to create an inherited event with PERF_SAMPLE_READ; this is deemed
acceptible because its results were always incorrect.

Reported-by: Andi Kleen <a...@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Cc: Alexander Shishkin <alexander.shish...@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <a...@kernel.org>
Cc: Jiri Olsa <jo...@kernel.org>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Stephane Eranian <eran...@google.com>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Vince Weaver <vi...@deater.net>
Fixes:  3dab77fb1bf8 ("perf: Rework/fix the whole read vs group stuff")
Link: 
http://lkml.kernel.org/r/20170530094512.dy2nljns2uq7q...@hirez.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mi...@kernel.org>
Signed-off-by: Sasha Levin <alexander.le...@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
 kernel/events/core.c |   15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4861,9 +4861,6 @@ static void perf_output_read_one(struct
        __output_copy(handle, values, n * sizeof(u64));
 }
 
-/*
- * XXX PERF_FORMAT_GROUP vs inherited events seems difficult.
- */
 static void perf_output_read_group(struct perf_output_handle *handle,
                            struct perf_event *event,
                            u64 enabled, u64 running)
@@ -4908,6 +4905,13 @@ static void perf_output_read_group(struc
 #define PERF_FORMAT_TOTAL_TIMES (PERF_FORMAT_TOTAL_TIME_ENABLED|\
                                 PERF_FORMAT_TOTAL_TIME_RUNNING)
 
+/*
+ * XXX PERF_SAMPLE_READ vs inherited events seems difficult.
+ *
+ * The problem is that its both hard and excessively expensive to iterate the
+ * child list, not to mention that its impossible to IPI the children running
+ * on another CPU, from interrupt/NMI context.
+ */
 static void perf_output_read(struct perf_output_handle *handle,
                             struct perf_event *event)
 {
@@ -7194,9 +7198,10 @@ perf_event_alloc(struct perf_event_attr
        local64_set(&hwc->period_left, hwc->sample_period);
 
        /*
-        * we currently do not support PERF_FORMAT_GROUP on inherited events
+        * We currently do not support PERF_SAMPLE_READ on inherited events.
+        * See perf_output_read().
         */
-       if (attr->inherit && (attr->read_format & PERF_FORMAT_GROUP))
+       if (attr->inherit && (attr->sample_type & PERF_SAMPLE_READ))
                goto err_ns;
 
        pmu = perf_init_event(event);


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