From mboxrd@z Thu Jan 1 00:00:00 1970 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 A5C09C001DE for ; Sun, 9 Jul 2023 14:55:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232805AbjGIOzk (ORCPT ); Sun, 9 Jul 2023 10:55:40 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44846 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230356AbjGIOzh (ORCPT ); Sun, 9 Jul 2023 10:55:37 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3D5A5126; Sun, 9 Jul 2023 07:55:36 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id C77DE60BCC; Sun, 9 Jul 2023 14:55:35 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3C145C433CB; Sun, 9 Jul 2023 14:55:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1688914535; bh=MlyiBlIvrWR+FMvlZMKgxNC3A519/M8R+ToVXS7wk+o=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=GUT8a9/MmGbPo2ZhR9kGpL7jpOKGNnaDiRkg97nAOghPZdA+whsj0t2yRQMINH7Ea ghSzVkoI0xK37ImQ9Xdyo4iK7NxW5OlIVbmDjLOMcJFP0rFjdYGc80l0IpPspfAb6d xbPPGGBQRvHi/Y6pwFXsQX6H69GX14CUh8t949Tu4MAXDoMcauzsQzakjCJbVWWUne oohvM0Sb1AWFhwKEhGL/WVwpxvAhjKHfbZ4cy/po8bvbauPx30htA+cNVKbr0oyS5N pXlEUK+9/2kKZiVFyLdWYuzIR9FhqEpt9padqyUB1WR76WsjKC8drRfqqSoYLIqsrO QcdX7LjqSeTTg== From: Sasha Levin To: linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: "Paul E. McKenney" , Sasha Levin , frederic@kernel.org, quic_neeraju@quicinc.com, joel@joelfernandes.org, josh@joshtriplett.org, boqun.feng@gmail.com, rcu@vger.kernel.org Subject: [PATCH AUTOSEL 6.4 2/4] rcu: Mark additional concurrent load from ->cpu_no_qs.b.exp Date: Sun, 9 Jul 2023 10:55:28 -0400 Message-Id: <20230709145530.512177-2-sashal@kernel.org> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20230709145530.512177-1-sashal@kernel.org> References: <20230709145530.512177-1-sashal@kernel.org> MIME-Version: 1.0 X-stable: review X-Patchwork-Hint: Ignore X-stable-base: Linux 6.4.2 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Paul E. McKenney" [ Upstream commit 9146eb25495ea8bfb5010192e61e3ed5805ce9ef ] The per-CPU rcu_data structure's ->cpu_no_qs.b.exp field is updated only on the instance corresponding to the current CPU, but can be read more widely. Unmarked accesses are OK from the corresponding CPU, but only if interrupts are disabled, given that interrupt handlers can and do modify this field. Unfortunately, although the load from rcu_preempt_deferred_qs() is always carried out from the corresponding CPU, interrupts are not necessarily disabled. This commit therefore upgrades this load to READ_ONCE. Similarly, the diagnostic access from synchronize_rcu_expedited_wait() might run with interrupts disabled and from some other CPU. This commit therefore marks this load with data_race(). Finally, the C-language access in rcu_preempt_ctxt_queue() is OK as is because interrupts are disabled and this load is always from the corresponding CPU. This commit adds a comment giving the rationale for this access being safe. This data race was reported by KCSAN. Not appropriate for backporting due to failure being unlikely. Signed-off-by: Paul E. McKenney Signed-off-by: Sasha Levin --- kernel/rcu/tree_exp.h | 2 +- kernel/rcu/tree_plugin.h | 4 +++- 2 files changed, 4 insertions(+), 2 deletions(-) diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h index 3b7abb58157df..8239b39d945bd 100644 --- a/kernel/rcu/tree_exp.h +++ b/kernel/rcu/tree_exp.h @@ -643,7 +643,7 @@ static void synchronize_rcu_expedited_wait(void) "O."[!!cpu_online(cpu)], "o."[!!(rdp->grpmask & rnp->expmaskinit)], "N."[!!(rdp->grpmask & rnp->expmaskinitnext)], - "D."[!!(rdp->cpu_no_qs.b.exp)]); + "D."[!!data_race(rdp->cpu_no_qs.b.exp)]); } } pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n", diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 7b0fe741a0886..41021080ad258 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -257,6 +257,8 @@ static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp) * GP should not be able to end until we report, so there should be * no need to check for a subsequent expedited GP. (Though we are * still in a quiescent state in any case.) + * + * Interrupts are disabled, so ->cpu_no_qs.b.exp cannot change. */ if (blkd_state & RCU_EXP_BLKD && rdp->cpu_no_qs.b.exp) rcu_report_exp_rdp(rdp); @@ -941,7 +943,7 @@ notrace void rcu_preempt_deferred_qs(struct task_struct *t) { struct rcu_data *rdp = this_cpu_ptr(&rcu_data); - if (rdp->cpu_no_qs.b.exp) + if (READ_ONCE(rdp->cpu_no_qs.b.exp)) rcu_report_exp_rdp(rdp); } -- 2.39.2