diff options
authorLinus Torvalds <torvalds@linux-foundation.org>2019-06-03 13:26:20 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2019-06-03 13:26:20 -0700
commit66be4e66a7f422128748e3c3ef6ee72b20a6197b (patch)
parent30d1d92a888d03681b927c76a35181b4eed7071f (diff)
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>
1 files changed, 2 insertions, 4 deletions
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 922bb6848813..b25d20822e75 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -56,14 +56,12 @@ void __rcu_read_unlock(void);
static inline void __rcu_read_lock(void)
- preempt_disable();
+ preempt_disable();
static inline void __rcu_read_unlock(void)
- preempt_enable();
+ preempt_enable();
static inline int rcu_preempt_depth(void)