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| author | Linus Torvalds <torvalds@linux-foundation.org> | 2019-06-03 13:26:20 -0700 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2019-06-11 12:24:12 +0200 |
| commit | f0d1e74c8120fe0a682a1caacca47333d95ba043 (patch) | |
| tree | 814f0553cc37a4f463746756288aa71b68266558 /include | |
| parent | 44657dbba7c4d176bf813998cd937e6987a3cac6 (diff) | |
| download | linux-f0d1e74c8120fe0a682a1caacca47333d95ba043.tar.gz linux-f0d1e74c8120fe0a682a1caacca47333d95ba043.tar.bz2 linux-f0d1e74c8120fe0a682a1caacca47333d95ba043.zip | |
rcu: locking and unlocking need to always be at least barriers
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>
Diffstat (limited to 'include')
| -rw-r--r-- | include/linux/rcupdate.h | 6 |
1 files changed, 2 insertions, 4 deletions
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index a0189ba67fde..addd03641e1a 100644 --- 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) |