linux-kernel - [PATCH 03/10] rcu/nocb: Remove needless LOAD-ACQUIRE

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20230908203603.5865-4-frederic@kernel.org>
Date:   Fri,  8 Sep 2023 22:35:56 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E . McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        rcu <rcu@...r.kernel.org>, Uladzislau Rezki <urezki@...il.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: [PATCH 03/10] rcu/nocb: Remove needless LOAD-ACQUIRE

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>
---
 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 b9eab359c597..6e63ba4788e1 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"));
 		}
-- 
2.41.0