[PATCH 1/6] rcu/nocb: Remove needless LOAD-ACQUIRE

Boqun Feng
2024-02-01T01:42:12Z

From: Frederic Weisbecker <frederic@kernel.org>

The LOAD-ACQUIRE access performed on rdp->nocb_cb_sleep advertizes
ordering callback execution against grace period completion. However
this is contradicted by the following:

* This LOAD-ACQUIRE doesn't pair with anything. The only counterpart
barrier that can be found is the smp_mb() placed after callbacks
advancing in nocb_gp_wait(). However the barrier is placed _after_
->nocb_cb_sleep write.

* Callbacks can be concurrently advanced between the LOAD-ACQUIRE on
->nocb_cb_sleep and the call to rcu_segcblist_extract_done_cbs() in
rcu_do_batch(), making any ordering based on ->nocb_cb_sleep broken.

* Both rcu_segcblist_extract_done_cbs() and rcu_advance_cbs() are called
under the nocb_lock, the latter hereby providing already the desired
ACQUIRE semantics.

Therefore it is safe to access ->nocb_cb_sleep with a simple compiler
barrier.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
kernel/rcu/tree_nocb.h | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 4efbf7333d4e..785946834c6b 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -933,8 +933,7 @@ static void nocb_cb_wait(struct rcu_data *rdp)
swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
nocb_cb_wait_cond(rdp));

- // VVV Ensure CB invocation follows _sleep test.
- if (smp_load_acquire(&rdp->nocb_cb_sleep)) { // ^^^
+ if (READ_ONCE(rdp->nocb_cb_sleep)) {
WARN_ON(signal_pending(current));
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WokeEmpty"));
}
--
2.43.0