[PATCH] perf mmap: Fix accessing unmapped mmap in perf_mmap__read_done()

[kan . liang]

From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>

There is segmentation fault when running perf trace. For example,
  [root@jouet e]# perf trace -e *chdir -o /tmp/bla perf report
  --ignore-vmlinux -i ../perf.data

The perf_mmap__consume() could unmap the mmap. It needs to check the
refcnt in perf_mmap__read_done().

Fixes: ee023de05f35 ("perf mmap: Introduce perf_mmap__read_done()")
Reported-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
---
 tools/perf/util/mmap.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/tools/perf/util/mmap.c b/tools/perf/util/mmap.c
index 38ca3ff..f6cfc52 100644
--- a/tools/perf/util/mmap.c
+++ b/tools/perf/util/mmap.c
@@ -317,5 +317,11 @@ int perf_mmap__push(struct perf_mmap *md, void *to,
  */
 void perf_mmap__read_done(struct perf_mmap *map)
 {
+	/*
+	 * Check if event was unmapped due to a POLLHUP/POLLERR.
+	 */
+	if (!refcount_read(&map->refcnt))
+		return;
+
 	map->prev = perf_mmap__read_head(map);
 }
-- 
2.4.11