perf evlist: Clarify a bit the use of perf_mmap->refcnt
This is an odd refcount use case, so add some more comments to help
understand that when it hits zero it really means that the mmap()ed area
(on a perf_event_open() returned fd) has been munmap()ed.
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Elena Reshetova <elena.reshetova@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/20170223162344.GD3595@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index 564b924..50420cd 100644
--- a/tools/perf/util/evlist.c
+++ b/tools/perf/util/evlist.c
@@ -974,8 +974,19 @@
if (!map)
return NULL;
- for (i = 0; i < evlist->nr_mmaps; i++)
+ for (i = 0; i < evlist->nr_mmaps; i++) {
map[i].fd = -1;
+ /*
+ * When the perf_mmap() call is made we grab one refcount, plus
+ * one extra to let perf_evlist__mmap_consume() get the last
+ * events after all real references (perf_mmap__get()) are
+ * dropped.
+ *
+ * Each PERF_EVENT_IOC_SET_OUTPUT points to this mmap and
+ * thus does perf_mmap__get() on it.
+ */
+ refcount_set(&map[i].refcnt, 0);
+ }
return map;
}