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perf evlist: Clarify a bit the use of perf_mmap->refcnt

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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 <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Elena Reshetova <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Wang Nan <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>
This commit is contained in:
Arnaldo Carvalho de Melo 2017-02-23 13:24:34 -03:00
commit 4738ca30b4
1 changed files with 12 additions and 1 deletions
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13
tools/perf/util/evlist.c
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@ -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;
}