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[ Upstream commit 0a6efab33eab4e973db26d9f90c3e97a7a82e399 ] On my device reading entirety of /sys/devices/pnp0/00:03/cmos_nvram0/nvmem takes about 9 msec during which time interrupts are off on the CPU that does the read and the thread that performs the read can not be migrated or preempted by another higher priority thread (RT or not). Allow readers and writers be preempted by taking and releasing rtc_lock spinlock for each individual byte read or written rather than once per read/write request. Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com> Reviewed-by: Mateusz Jończyk <mat.jonczyk@o2.pl> Link: https://lore.kernel.org/r/Zxv8QWR21AV4ztC5@google.com Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com> Signed-off-by: Sasha Levin <sashal@kernel.org>