From: Linus Torvalds Date: Mon, 3 Jun 2019 20:26:20 +0000 (-0700) Subject: rcu: locking and unlocking need to always be at least barriers X-Git-Tag: v4.4.181~10 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=f0d1e74c8120fe0a682a1caacca47333d95ba043;p=thirdparty%2Fkernel%2Fstable.git rcu: locking and unlocking need to always be at least barriers commit 66be4e66a7f422128748e3c3ef6ee72b20a6197b upstream. 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 Fixes: bb73c52bad36 ("rcu: Don't disable preemption for Tiny and Tree RCU readers") Cc: stable@kernel.org Cc: Boqun Feng Cc: Paul E. McKenney Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index a0189ba67fde7..addd03641e1ab 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -297,14 +297,12 @@ void synchronize_rcu(void); static inline void __rcu_read_lock(void) { - if (IS_ENABLED(CONFIG_PREEMPT_COUNT)) - preempt_disable(); + preempt_disable(); } static inline void __rcu_read_unlock(void) { - if (IS_ENABLED(CONFIG_PREEMPT_COUNT)) - preempt_enable(); + preempt_enable(); } static inline void synchronize_rcu(void)