linux-kernel - [tip:perf/core] perf evlist: Clarify a bit the use of perf

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Message-ID: <tip-4738ca30b4a7a113084d7863846175094f95c62f@git.kernel.org>
Date:   Tue, 7 Mar 2017 00:02:57 -0800
From:   tip-bot for Arnaldo Carvalho de Melo <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     acme@...hat.com, dsahern@...il.com, mingo@...nel.org,
        elena.reshetova@...el.com, tglx@...utronix.de, wangnan0@...wei.com,
        adrian.hunter@...el.com, linux-kernel@...r.kernel.org,
        jolsa@...nel.org, peterz@...radead.org, namhyung@...nel.org,
        hpa@...or.com
Subject: [tip:perf/core] perf evlist: Clarify a bit the use of
 perf_mmap->refcnt

Commit-ID:  4738ca30b4a7a113084d7863846175094f95c62f
Gitweb:     http://git.kernel.org/tip/4738ca30b4a7a113084d7863846175094f95c62f
Author:     Arnaldo Carvalho de Melo <acme@...hat.com>
AuthorDate: Thu, 23 Feb 2017 13:24:34 -0300
Committer:  Arnaldo Carvalho de Melo <acme@...hat.com>
CommitDate: Fri, 3 Mar 2017 19:07:16 -0300

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@...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 564b924..50420cd 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;
 }