From: Linus Torvalds torvalds@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 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 Signed-off-by: Greg Kroah-Hartman gregkh@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)