# git rev-parse -q --verify 66be4e66a7f422128748e3c3ef6ee72b20a6197b^{commit}
66be4e66a7f422128748e3c3ef6ee72b20a6197b
already have revision, skipping fetch
# git checkout -q -f -B kisskb 66be4e66a7f422128748e3c3ef6ee72b20a6197b
# git clean -qxdf
# < git log -1
# commit 66be4e66a7f422128748e3c3ef6ee72b20a6197b
# Author: Linus Torvalds <torvalds@linux-foundation.org>
# Date:   Mon Jun 3 13:26:20 2019 -0700
# 
#     rcu: locking and unlocking need to always be at least barriers
#     
#     Herbert Xu pointed out that commit bb73c52bad36 ("rcu: Don't disable
#     preemption for Tiny and Tree RCU readers") was incorrect in making the
#     preempt_disable/enable() be conditional on CONFIG_PREEMPT_COUNT.
#     
#     If CONFIG_PREEMPT_COUNT isn't enabled, the preemption enable/disable is
#     a no-op, but still is a compiler barrier.
#     
#     And RCU locking still _needs_ that compiler barrier.
#     
#     It is simply fundamentally not true that RCU locking would be a complete
#     no-op: we still need to guarantee (for example) that things that can
#     trap and cause preemption cannot migrate into the RCU locked region.
#     
#     The way we do that is by making it a barrier.
#     
#     See for example commit 386afc91144b ("spinlocks and preemption points
#     need to be at least compiler barriers") from back in 2013 that had
#     similar issues with spinlocks that become no-ops on UP: they must still
#     constrain the compiler from moving other operations into the critical
#     region.
#     
#     Now, it is true that a lot of RCU operations already use READ_ONCE() and
#     WRITE_ONCE() (which in practice likely would never be re-ordered wrt
#     anything remotely interesting), but it is also true that that is not
#     globally the case, and that it's not even necessarily always possible
#     (ie bitfields etc).
#     
#     Reported-by: Herbert Xu <herbert@gondor.apana.org.au>
#     Fixes: bb73c52bad36 ("rcu: Don't disable preemption for Tiny and Tree RCU readers")
#     Cc: stable@kernel.org
#     Cc: Boqun Feng <boqun.feng@gmail.com>
#     Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
#     Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
# < /opt/cross/kisskb/korg/gcc-8.1.0-nolibc/mips-linux/bin/mips-linux-gcc --version
# < /opt/cross/kisskb/korg/gcc-8.1.0-nolibc/mips-linux/bin/mips-linux-ld --version
# < git log --format=%s --max-count=1 66be4e66a7f422128748e3c3ef6ee72b20a6197b
# < make -s -j 48 ARCH=mips O=/kisskb/build/linus_malta_defconfig_mips-gcc8 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-8.1.0-nolibc/mips-linux/bin/mips-linux-  malta_defconfig
# make -s -j 48 ARCH=mips O=/kisskb/build/linus_malta_defconfig_mips-gcc8 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-8.1.0-nolibc/mips-linux/bin/mips-linux-  
Completed OK
# rm -rf /kisskb/build/linus_malta_defconfig_mips-gcc8
# Build took: 0:02:15.407961