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[PATCH 19/35] perf evlist: Clarify a bit the use of perf_mmap->refcnt

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From: Arnaldo Carvalho de Melo <acme@redhat.com>

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>
---
 tools/perf/util/evlist.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index 564b924fb48a..50420cd35446 100644
--- a/tools/perf/util/evlist.c
+++ b/tools/perf/util/evlist.c
@@ -974,8 +974,19 @@ static struct perf_mmap *perf_evlist__alloc_mmap(struct perf_evlist *evlist)
 	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;
 }
 
-- 
2.9.3

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[GIT PULL 00/35] perf/core improvements and fixes Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 22/35] perf cpumap: Introduce cpu_map__snprint_mask() Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 12/35] perf comm: Convert comm_str.refcnt from atomic_t to refcount_t Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 30/35] perf tools: Force uncore events to system wide monitoring Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 27/35] perf probe: Generalize probe event file open routine Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 17/35] perf thread: convert thread.refcnt from atomic_t to refcount_t Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 25/35] kretprobes: Ensure probe location is at function entry Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 13/35] perf dso: Convert dso.refcnt from atomic_t to refcount_t Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 21/35] perf ftrace: Add support for --pid option Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 19/35] perf evlist: Clarify a bit the use of perf_mmap->refcnt Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 23/35] perf ftrace: Add support for -a and -C option Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 10/35] perf cgroup: Convert cgroup_sel.refcnt from atomic_t to refcount_t Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 07/35] tools include: Provide gcc based cmpxchg fallback for !x86 Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 20/35] perf tools: Allow sorting by symbol size Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 28/35] perf intel-PT/BTS: Add missing initialization Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 33/35] perf bench futex: Fix build on musl + clang Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 05/35] tools arch x86: Introduce atomic_cmpxchg() Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 14/35] perf map: Convert map.refcnt from atomic_t to refcount_t Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 16/35] perf evlist: Convert perf_map.refcnt from atomic_t to refcount_t Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 06/35] tools include: Introduce atomic_cmpxchg_{relaxed,release}() Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 24/35] perf ftrace: Use pager for displaying result Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 35/35] perf bench numa: Add more comment for -c option Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 11/35] perf cpumap: Convert cpu_map.refcnt from atomic_t to refcount_t Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 02/35] perf stat: Issue a HW watchdog disable hint Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 26/35] trace/kprobes: Allow return probes with offsets and absolute addresses Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 15/35] perf map: Convert map_groups.refcnt from atomic_t to refcount_t Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 01/35] perf vendor events: Add mapping for KnightsMill PMU events Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 32/35] perf bench futex: Use __maybe_unused Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 29/35] trace/kprobes: Add back warning about offset in return probes Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 31/35] tools build: Add test for sched_getcpu() Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 09/35] tools include: Adopt kernel's refcount.h Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 18/35] perf thread_map: Convert thread_map.refcnt from atomic_t to refcount_t Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  [PATCH 34/35] tools build: Use the same CC for feature detection and actual build Arnaldo Carvalho de Melo <acme@kernel.org> - 2017-03-06 20:50 +0100
  Re: [GIT PULL 00/35] perf/core improvements and fixes Ingo Molnar <mingo@kernel.org> - 2017-03-07 08:20 +0100

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