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

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Date: Wed, 31 Jan 2024 17:40:53 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: linux-kernel@...r.kernel.org,
	rcu@...r.kernel.org
Cc: Frederic Weisbecker <frederic@...nel.org>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Neeraj Upadhyay <quic_neeraju@...cinc.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>
Subject: [PATCH 1/6] rcu/nocb: Remove needless LOAD-ACQUIRE

From: Frederic Weisbecker <frederic@...nel.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@...nel.org>
Reviewed-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Boqun Feng <boqun.feng@...il.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