Commit f0d1e74c authored by Linus Torvalds's avatar Linus Torvalds Committed by Greg Kroah-Hartman
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rcu: locking and unlocking need to always be at least barriers

commit 66be4e66a7f422128748e3c3ef6ee72b20a6197b upstream.

Herbert Xu pointed out that commit bb73c52b ("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 386afc91

 ("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

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: default avatarHerbert Xu <>
Fixes: bb73c52b

 ("rcu: Don't disable preemption for Tiny and Tree RCU readers")
Cc: Boqun Feng <>
Cc: Paul E. McKenney <>
Signed-off-by: default avatarLinus Torvalds <>
Signed-off-by: default avatarGreg Kroah-Hartman <>
parent 44657dbb
......@@ -297,14 +297,12 @@ void synchronize_rcu(void);
static inline void __rcu_read_lock(void)
static inline void __rcu_read_unlock(void)
static inline void synchronize_rcu(void)
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