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 2218EC433EF for ; Mon, 17 Jan 2022 17:14:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244019AbiAQROV (ORCPT ); Mon, 17 Jan 2022 12:14:21 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47304 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S243759AbiAQRKW (ORCPT ); Mon, 17 Jan 2022 12:10:22 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B6C06C0698C1; Mon, 17 Jan 2022 09:06:08 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 76127B81147; Mon, 17 Jan 2022 17:06:07 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 29C6EC36AEC; Mon, 17 Jan 2022 17:06:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1642439166; bh=fqghv2tM9KQdZtwnpamw7ZoE6favq2oByT6WYmfaJKk=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=h/LGIgJO+n9cEKguUMgUtfXajjC2aNK+ndKdcEV7r9zbQZNND7g2tu8N65Q8RbYCC ktJDG5cwVkmFUv6dkCZC8ohsojSGSB0LXFbgGlKo47lwvFTlEwXnwPWsYRedWFqRas DRLN3tW68bIYSX2UoJmSEf5OumVqR0w3YegXTw1inuVDf8SZgRNI5ZYNU59znoAqv0 sz868a9RWULaCAkAAMUdRcDpEmG1tqcoJHyzbJY/J2Rm8sTLSPUqNKrsP1hVro3vP6 xkxuGddl53ByrP1ZvFkcHqHVxLIdq8S9nk6bccPY1jEqtsqIt7p+4MNhOSrFC6nhxS WgsafsSM8QJtQ== From: Sasha Levin To: linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: Nicholas Piggin , Laurent Dufour , Michael Ellerman , Sasha Levin , pmladek@suse.com, john.ogness@linutronix.de, clg@kaod.org, sudeep.holla@arm.com, linuxppc-dev@lists.ozlabs.org Subject: [PATCH AUTOSEL 4.19 06/17] powerpc/watchdog: Fix missed watchdog reset due to memory ordering race Date: Mon, 17 Jan 2022 12:05:40 -0500 Message-Id: <20220117170551.1472640-6-sashal@kernel.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20220117170551.1472640-1-sashal@kernel.org> References: <20220117170551.1472640-1-sashal@kernel.org> MIME-Version: 1.0 X-stable: review X-Patchwork-Hint: Ignore Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Nicholas Piggin [ Upstream commit 5dad4ba68a2483fc80d70b9dc90bbe16e1f27263 ] It is possible for all CPUs to miss the pending cpumask becoming clear, and then nobody resetting it, which will cause the lockup detector to stop working. It will eventually expire, but watchdog_smp_panic will avoid doing anything if the pending mask is clear and it will never be reset. Order the cpumask clear vs the subsequent test to close this race. Add an extra check for an empty pending mask when the watchdog fires and finds its bit still clear, to try to catch any other possible races or bugs here and keep the watchdog working. The extra test in arch_touch_nmi_watchdog is required to prevent the new warning from firing off. Signed-off-by: Nicholas Piggin Reviewed-by: Laurent Dufour Debugged-by: Laurent Dufour Signed-off-by: Michael Ellerman Link: https://lore.kernel.org/r/20211110025056.2084347-2-npiggin@gmail.com Signed-off-by: Sasha Levin --- arch/powerpc/kernel/watchdog.c | 41 +++++++++++++++++++++++++++++++++- 1 file changed, 40 insertions(+), 1 deletion(-) diff --git a/arch/powerpc/kernel/watchdog.c b/arch/powerpc/kernel/watchdog.c index af3c15a1d41eb..75b2a6c4db5a5 100644 --- a/arch/powerpc/kernel/watchdog.c +++ b/arch/powerpc/kernel/watchdog.c @@ -132,6 +132,10 @@ static void set_cpumask_stuck(const struct cpumask *cpumask, u64 tb) { cpumask_or(&wd_smp_cpus_stuck, &wd_smp_cpus_stuck, cpumask); cpumask_andnot(&wd_smp_cpus_pending, &wd_smp_cpus_pending, cpumask); + /* + * See wd_smp_clear_cpu_pending() + */ + smp_mb(); if (cpumask_empty(&wd_smp_cpus_pending)) { wd_smp_last_reset_tb = tb; cpumask_andnot(&wd_smp_cpus_pending, @@ -217,13 +221,44 @@ static void wd_smp_clear_cpu_pending(int cpu, u64 tb) cpumask_clear_cpu(cpu, &wd_smp_cpus_stuck); wd_smp_unlock(&flags); + } else { + /* + * The last CPU to clear pending should have reset the + * watchdog so we generally should not find it empty + * here if our CPU was clear. However it could happen + * due to a rare race with another CPU taking the + * last CPU out of the mask concurrently. + * + * We can't add a warning for it. But just in case + * there is a problem with the watchdog that is causing + * the mask to not be reset, try to kick it along here. + */ + if (unlikely(cpumask_empty(&wd_smp_cpus_pending))) + goto none_pending; } return; } + cpumask_clear_cpu(cpu, &wd_smp_cpus_pending); + + /* + * Order the store to clear pending with the load(s) to check all + * words in the pending mask to check they are all empty. This orders + * with the same barrier on another CPU. This prevents two CPUs + * clearing the last 2 pending bits, but neither seeing the other's + * store when checking if the mask is empty, and missing an empty + * mask, which ends with a false positive. + */ + smp_mb(); if (cpumask_empty(&wd_smp_cpus_pending)) { unsigned long flags; +none_pending: + /* + * Double check under lock because more than one CPU could see + * a clear mask with the lockless check after clearing their + * pending bits. + */ wd_smp_lock(&flags); if (cpumask_empty(&wd_smp_cpus_pending)) { wd_smp_last_reset_tb = tb; @@ -314,8 +349,12 @@ void arch_touch_nmi_watchdog(void) { unsigned long ticks = tb_ticks_per_usec * wd_timer_period_ms * 1000; int cpu = smp_processor_id(); - u64 tb = get_tb(); + u64 tb; + if (!cpumask_test_cpu(cpu, &watchdog_cpumask)) + return; + + tb = get_tb(); if (tb - per_cpu(wd_timer_tb, cpu) >= ticks) { per_cpu(wd_timer_tb, cpu) = tb; wd_smp_clear_cpu_pending(cpu, tb); -- 2.34.1