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

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Message-Id: <20190609164155.511875733@linuxfoundation.org>
Date:   Sun,  9 Jun 2019 18:42:56 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Herbert Xu <herbert@...dor.apana.org.au>,
        stable@...nel.org, Boqun Feng <boqun.feng@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 4.4 234/241] rcu: locking and unlocking need to always be at least barriers

From: Linus Torvalds <torvalds@...ux-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@...dor.apana.org.au>
Fixes: bb73c52bad36 ("rcu: Don't disable preemption for Tiny and Tree RCU readers")
Cc: stable@...nel.org
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 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)