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McKenney" Subject: [PATCH 2/3] rcu/nocb: Remove needless LOAD-ACQUIRE Date: Wed, 15 Nov 2023 14:11:27 -0500 Message-ID: <20231115191128.15615-3-frederic@kernel.org> X-Mailer: git-send-email 2.42.1 In-Reply-To: <20231115191128.15615-1-frederic@kernel.org> References: <20231115191128.15615-1-frederic@kernel.org> MIME-Version: 1.0 X-Spam-Status: No, score=-1.3 required=5.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,T_SCC_BODY_TEXT_LINE autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lipwig.vger.email Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-Greylist: Sender passed SPF test, not delayed by milter-greylist-4.6.4 (lipwig.vger.email [0.0.0.0]); Wed, 15 Nov 2023 11:11:57 -0800 (PST) X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1782658391046563042 X-GMAIL-MSGID: 1782658391046563042 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 --- 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 aecef51166c7..eb27878d46f1 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")); }