perf tools: Fix race in build_id_cache__add_s()

int build_id_cache__add_s(const char *sbuild_id, const char *debugdir,
                          const char *name, bool is_kallsyms, bool is_vdso)
{
...
        if (access(filename, F_OK)) {
               ^--------------------------------------------------------- [1]
                if (is_kallsyms) {
                         if (copyfile("/proc/kallsyms", filename))
                                goto out_free;
                } else if (link(realname, filename) && copyfile(name, filename))
                             ^-----------------------------^------------- [2]
                                                            \------------ [3]
                        goto out_free;
        }
...

When multiple instances of perf record get to [1] at more or less same time and
run access() one or more may get failure because the file does not exist yet
(since the first instance did not have chance to link it yet).

At this point the race moves to link() at [2] where first thread to get
there links file and goes on but second one gets -EEXIST so it runs
copyfile [3] which truncates the file.

reproducer:

rm -rf /root/.debug
for cpu in $(awk '/processor/ {print $3}' /proc/cpuinfo); do
	perf record -a -v -T -F 1000 -C $cpu \
		-o perf-${cpu}.data sleep 5 2> /dev/null &
done
wait

and simply search for empty files by:

find /lib/modules/`uname -r`/kernel/* -size 0
Signed-off-by: Milos Vyletel <milos@redhat.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1426847846-11112-1-git-send-email-milos@redhat.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/build-id.c

@@ -374,7 +374,8 @@ int build_id_cache__add_s(const char *sbuild_id, const char *name,
 		if (is_kallsyms) {
 			 if (copyfile("/proc/kallsyms", filename))
 				goto out_free;
-		} else if (link(realname, filename) && copyfile(name, filename))
+		} else if (link(realname, filename) && errno != EEXIST &&
+				copyfile(name, filename))
 			goto out_free;
 	}