From: Linus Torvalds <torva...@linux-foundation.org>
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 <herb...@gondor.apana.org.au>
Fixes: bb73c52bad36 ("rcu: Don't disable preemption for Tiny and Tree RCU
readers")
Cc: sta...@kernel.org
Cc: Boqun Feng <boqun.f...@gmail.com>
Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torva...@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
include/linux/rcupdate.h | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -306,14 +306,12 @@ 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)
