From patchwork Wed Nov 15 19:11:28 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Frederic Weisbecker X-Patchwork-Id: 13457246 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 219A4C2BB3F for ; Wed, 15 Nov 2023 19:11:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230183AbjKOTLl (ORCPT ); Wed, 15 Nov 2023 14:11:41 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36272 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232757AbjKOTLi (ORCPT ); Wed, 15 Nov 2023 14:11:38 -0500 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 18CE2FA; Wed, 15 Nov 2023 11:11:35 -0800 (PST) Received: by smtp.kernel.org (Postfix) with ESMTPSA id D9CFCC433CA; Wed, 15 Nov 2023 19:11:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1700075494; bh=+CwFhpZFJagUYR6sRV+zAT+zjPxnRMlVxvZF4rtuuss=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=pRqQ355MpVkvPG4Din2KnuRYnR/dMc2x1hGi2P6G7B+yVZ2hOFIBMQ4lM3iTqMSXE RJW6mwFeShgda59qG3xg/J3BomgtT4vSBO/ZEL6/FvYY9DK8HUDM/cfcJSEyWOh6CG Hoputu2RO/KdslIEI5HYGubKKLr3tPFbzt4Z+qYBkob18Fma8mEn+Rnwt3viklXZio zA1ZVP9AwOV/D2JIpJ4fMqNutCtaEcFYOSFVe5NubGQfEJGZpCqMWTZOxMSFmK09Kk svMEGvysR9VqV2n68d/M+7+Jdwhh4ahR3lfOpoAfGM2IMbsEBbJsyNoYMoSHaJKnYz YhIHx16iU/LBQ== From: Frederic Weisbecker To: LKML Cc: Frederic Weisbecker , Boqun Feng , Joel Fernandes , Neeraj Upadhyay , Uladzislau Rezki , Zqiang , rcu , "Paul E. McKenney" Subject: [PATCH 3/3] rcu/nocb: Remove needless full barrier after callback advancing Date: Wed, 15 Nov 2023 14:11:28 -0500 Message-ID: <20231115191128.15615-4-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 Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org A full barrier is issued from nocb_gp_wait() upon callbacks advancing to order grace period completion with callbacks execution. However these two events are already ordered by the smp_mb__after_unlock_lock() barrier within the call to raw_spin_lock_rcu_node() that is necessary for callbacks advancing to happen. The following litmus test shows the kind of guarantee that this barrier provides: C smp_mb__after_unlock_lock {} // rcu_gp_cleanup() P0(spinlock_t *rnp_lock, int *gpnum) { // Grace period cleanup increase gp sequence number spin_lock(rnp_lock); WRITE_ONCE(*gpnum, 1); spin_unlock(rnp_lock); } // nocb_gp_wait() P1(spinlock_t *rnp_lock, spinlock_t *nocb_lock, int *gpnum, int *cb_ready) { int r1; // Call rcu_advance_cbs() from nocb_gp_wait() spin_lock(nocb_lock); spin_lock(rnp_lock); smp_mb__after_unlock_lock(); r1 = READ_ONCE(*gpnum); WRITE_ONCE(*cb_ready, 1); spin_unlock(rnp_lock); spin_unlock(nocb_lock); } // nocb_cb_wait() P2(spinlock_t *nocb_lock, int *cb_ready, int *cb_executed) { int r2; // rcu_do_batch() -> rcu_segcblist_extract_done_cbs() spin_lock(nocb_lock); r2 = READ_ONCE(*cb_ready); spin_unlock(nocb_lock); // Actual callback execution WRITE_ONCE(*cb_executed, 1); } P3(int *cb_executed, int *gpnum) { int r3; WRITE_ONCE(*cb_executed, 2); smp_mb(); r3 = READ_ONCE(*gpnum); } exists (1:r1=1 /\ 2:r2=1 /\ cb_executed=2 /\ 3:r3=0) (* Bad outcome. *) Here the bad outcome only occurs if the smp_mb__after_unlock_lock() is removed. This barrier orders the grace period completion against callbacks advancing and even later callbacks invocation, thanks to the opportunistic propagation via the ->nocb_lock to nocb_cb_wait(). Therefore the smp_mb() placed after callbacks advancing can be safely removed. Signed-off-by: Frederic Weisbecker --- kernel/rcu/tree.c | 6 ++++++ kernel/rcu/tree_nocb.h | 1 - 2 files changed, 6 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 3ac3c846105f..c557302fc4f5 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2113,6 +2113,12 @@ static void rcu_do_batch(struct rcu_data *rdp) * Extract the list of ready callbacks, disabling IRQs to prevent * races with call_rcu() from interrupt handlers. Leave the * callback counts, as rcu_barrier() needs to be conservative. + * + * Callbacks execution is fully ordered against preceding grace period + * completion (materialized by rnp->gp_seq update) thanks to the + * smp_mb__after_unlock_lock() upon node locking required for callbacks + * advancing. In NOCB mode this ordering is then further relayed through + * the nocb locking that protects both callbacks advancing and extraction. */ rcu_nocb_lock_irqsave(rdp, flags); WARN_ON_ONCE(cpu_is_offline(smp_processor_id())); diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index eb27878d46f1..d82f96a66600 100644 --- a/kernel/rcu/tree_nocb.h +++ b/kernel/rcu/tree_nocb.h @@ -779,7 +779,6 @@ static void nocb_gp_wait(struct rcu_data *my_rdp) if (rcu_segcblist_ready_cbs(&rdp->cblist)) { needwake = rdp->nocb_cb_sleep; WRITE_ONCE(rdp->nocb_cb_sleep, false); - smp_mb(); /* CB invocation -after- GP end. */ } else { needwake = false; }