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rcu: locking and unlocking need to always be at least barriers

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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>
This commit is contained in:
Linus Torvalds 2019-06-03 13:26:20 -07:00 committed by Greg Kroah-Hartman
parent 39e597d283
commit 6726307d20
1 changed files with 2 additions and 4 deletions
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6
include/linux/rcupdate.h
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@ -78,14 +78,12 @@ 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)