This was introduced by v2.6.34-rc1~38:

	4c014e87 (rtc/mc13783: protect rtc {,un}registration by mc13783 lock)
Signed-off-by: NUwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reported-by: NDan Carpenter <error27@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>