linux-kernel - [PATCH 19/35] perf evlist: Clarify a bit the use of perf

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Message-Id: <20170306193825.24011-20-acme@kernel.org>
Date:   Mon,  6 Mar 2017 16:38:09 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        David Ahern <dsahern@...il.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Wang Nan <wangnan0@...wei.com>
Subject: [PATCH 19/35] perf evlist: Clarify a bit the use of perf_mmap->refcnt

From: Arnaldo Carvalho de Melo <acme@...hat.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@...el.com>
Cc: David Ahern <dsahern@...il.com>
Cc: Elena Reshetova <elena.reshetova@...el.com>
Cc: Jiri Olsa <jolsa@...nel.org>
Cc: Namhyung Kim <namhyung@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Wang Nan <wangnan0@...wei.com>
Link: http://lkml.kernel.org/r/20170223162344.GD3595@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.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