Files
linux/include/linux
Linus Torvalds c8baeecd2d rcu: locking and unlocking need to always be at least barriers
commit 66be4e66a7 upstream.

Herbert Xu pointed out that commit bb73c52bad ("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 386afc9114 ("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: bb73c52bad ("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>
2023-05-15 13:40:55 +09:00
..
2023-04-21 13:52:38 +09:00
2015-10-07 18:08:15 +01:00
2023-05-15 12:22:38 +09:00
2016-07-08 16:23:11 +02:00
2016-08-25 11:26:48 -04:00
2023-05-15 12:23:21 +09:00
2017-10-08 10:26:06 +02:00
2018-04-24 09:34:18 +02:00
2018-04-12 09:54:41 -07:00
2018-07-03 18:27:19 +02:00
2016-06-07 13:41:38 -06:00
2016-08-11 09:41:35 -06:00
2018-03-31 18:42:34 +02:00
2017-06-14 15:06:00 +02:00
2018-07-03 18:27:19 +02:00
2018-02-28 10:18:33 +01:00
2018-05-30 13:19:56 +02:00
2023-05-15 12:06:31 +09:00
2017-07-15 12:16:11 +02:00
2016-06-07 13:41:38 -06:00
2016-05-17 15:48:12 -04:00
2018-04-12 09:54:41 -07:00
2017-07-12 15:01:02 +02:00
2017-12-10 17:13:13 +01:00
2018-04-12 09:54:41 -07:00
2018-03-14 20:21:31 -08:00
2016-07-22 09:07:02 +02:00
2016-09-24 10:48:18 +02:00
2015-11-25 09:22:00 -07:00
2016-02-11 09:59:22 -05:00
2018-04-09 11:39:17 -07:00
2016-10-20 15:51:28 +11:00
2016-09-16 09:34:15 +01:00
2016-09-14 09:18:09 -06:00
2018-01-02 20:45:15 +01:00
2016-05-11 22:37:54 +02:00
2015-06-25 12:06:45 +02:00
2016-01-28 14:19:12 -08:00
2023-05-15 08:33:22 +09:00
2016-08-10 11:23:44 -04:00
2017-10-30 09:27:09 +01:00
2016-03-22 15:36:02 -07:00
2015-07-28 08:50:42 +01:00
2016-01-15 17:56:32 -08:00
2016-09-15 16:49:39 +02:00
2016-09-27 12:33:47 +02:00
2017-08-24 17:12:19 -07:00
2018-04-03 17:37:41 -08:00
2018-04-17 17:58:08 -08:00
2015-06-24 17:49:41 -07:00
2015-07-21 10:39:05 -07:00
2016-08-28 23:32:41 -04:00
2016-04-25 15:09:11 -04:00
2016-02-16 13:04:58 -05:00
2016-10-19 11:36:22 -06:00
2016-05-02 09:00:56 -05:00
2018-03-22 09:54:47 +01:00
2016-02-11 18:35:48 -08:00
2016-03-14 15:43:11 -04:00
2017-08-24 17:12:21 -07:00
2016-10-14 11:36:59 -07:00
2016-07-12 19:25:38 -07:00
2016-09-27 21:52:00 -04:00
2016-09-08 15:01:10 -07:00
2016-03-17 15:09:34 -07:00
2016-07-06 10:51:14 +01:00
2016-03-22 15:36:02 -07:00
2016-07-26 16:19:19 -07:00
2016-09-08 22:15:25 -07:00
2017-08-30 10:21:40 +02:00
2016-08-28 23:44:55 -04:00
2016-10-05 18:23:36 -04:00
2023-05-15 12:21:56 +09:00
2018-05-30 13:19:56 +02:00
2023-05-15 09:23:01 +09:00
2018-06-06 18:34:12 +02:00
2017-03-17 13:14:32 +08:00
2016-06-20 12:47:15 -07:00
2015-10-01 09:57:59 -07:00
2016-07-19 17:43:38 +03:00
2016-05-23 17:04:14 -07:00
2023-05-15 09:51:54 +09:00
2016-04-07 16:53:29 -04:00
2017-04-21 09:31:21 +02:00
2015-11-23 09:44:58 +01:00
2016-07-26 16:19:19 -07:00
2016-05-20 17:58:30 -07:00
2017-12-25 14:23:37 +01:00
2018-04-03 17:37:41 -08:00
2016-06-25 09:04:48 -07:00
2016-09-30 10:54:03 +02:00
2015-12-03 07:24:29 -08:00
2023-05-15 10:05:34 +09:00
2015-09-08 15:35:28 -07:00