[PATCH 4.4 234/241] rcu: locking and unlocking need to always be at least barriers

[Greg Kroah-Hartman]

From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

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 <herbert@xxxxxxxxxxxxxxxxxxx>
Fixes: bb73c52bad36 ("rcu: Don't disable preemption for Tiny and Tree RCU readers")
Cc: stable@xxxxxxxxxx
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 include/linux/rcupdate.h |    6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

--- 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)