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 47C37EB64DD for ; Sun, 2 Jul 2023 20:02:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233264AbjGBUCp (ORCPT ); Sun, 2 Jul 2023 16:02:45 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47762 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233231AbjGBUCa (ORCPT ); Sun, 2 Jul 2023 16:02:30 -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 7977761B9; Sun, 2 Jul 2023 12:59:17 -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 72FA960C96; Sun, 2 Jul 2023 19:57:55 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id D2D17C433C7; Sun, 2 Jul 2023 19:57:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1688327874; bh=4aGYSBKwgNAAIWotlDYiWF09w4v3buff4O6Htx71YHw=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=LnHDSR/B93C13iOIbCZ/q0oPLuydDGHETkgo2sZhxGW5aBZ30yo5kpQL2Fr1mzqxw Yl4JeYie7xpoQG3cSozgwdAYqhgzcQDRq81NWJ8bs9VpaZNlVWQUCqCpX8uUlJvncM wS/cIehuB30if/mmcNPTu4h3w69f4dzYcinvk4kcbPdIx0eLnSM5dlBrlfzI+LZowK DNDh4ktblaJBSF9eCnf7lNnWrsB6QEgVwKq3gH65Sf3vL/lC36ZTGJeZ7O/Mj8FFre 2rNnn9GsCwwHGHKIqT5Zdx59Csq7M9Kh0Ay0LSI5XtM2ODJRDsAYiMAu1ITV3pOGy/ qkCckil1R3beQ== 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 6.3 2/3] rcu: Mark additional concurrent load from ->cpu_no_qs.b.exp Date: Sun, 2 Jul 2023 15:57:49 -0400 Message-Id: <20230702195750.1793345-2-sashal@kernel.org> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20230702195750.1793345-1-sashal@kernel.org> References: <20230702195750.1793345-1-sashal@kernel.org> MIME-Version: 1.0 X-stable: review X-Patchwork-Hint: Ignore X-stable-base: Linux 6.3.11 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 54e8fb258c98d..3ee057b41cbb2 100644 --- a/kernel/rcu/tree_exp.h +++ b/kernel/rcu/tree_exp.h @@ -642,7 +642,7 @@ 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