From nobody Fri Sep 12 05:39:17 2025 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 B76F1C197A0 for ; Mon, 20 Nov 2023 08:36:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232228AbjKTIgk (ORCPT ); Mon, 20 Nov 2023 03:36:40 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41018 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231948AbjKTIgh (ORCPT ); Mon, 20 Nov 2023 03:36:37 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B1F6B9E for ; Mon, 20 Nov 2023 00:36:29 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1700469388; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=CBLqoMyIVireLlLreZE4y9YWUgoFtH3j+ZLwtIAR6bY=; b=HUHlzKLEfszrbruskvsw0HcBledqTaQAxNszDt+ZaAyPkPwt+ILYeigKpeMkiiYq8ehTu0 qsgsRfeqQGaxLj1iZ7WchGAjqR0jR+t5ons/C5sfEOSjtQ2ZSsI8mPxUYephkuOqNPoROs tmVIlUc6TSsjlEKNHHHj8iUVfjsl00Q= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-461-W8IigMUsOXuqc3NP8N005A-1; Mon, 20 Nov 2023 03:36:23 -0500 X-MC-Unique: W8IigMUsOXuqc3NP8N005A-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 90AA53C0E657; Mon, 20 Nov 2023 08:36:22 +0000 (UTC) Received: from localhost (unknown [10.72.120.15]) by smtp.corp.redhat.com (Postfix) with ESMTP id 8BDEA40C6EB9; Mon, 20 Nov 2023 08:36:21 +0000 (UTC) From: Ming Lei To: Thomas Gleixner , Andrew Morton Cc: linux-kernel@vger.kernel.org, Ming Lei , Keith Busch , linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, Yi Zhang , Guangwu Zhang , Chengming Zhou , Jens Axboe Subject: [PATCH V4 resend] lib/group_cpus.c: avoid to acquire cpu hotplug lock in group_cpus_evenly Date: Mon, 20 Nov 2023 16:35:59 +0800 Message-ID: <20231120083559.285174-1-ming.lei@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.2 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" group_cpus_evenly() could be part of storage driver's error handler, such as nvme driver, when may happen during CPU hotplug, in which storage queue has to drain its pending IOs because all CPUs associated with the queue are offline and the queue is becoming inactive. And handling IO needs error handler to provide forward progress. Then dead lock is caused: 1) inside CPU hotplug handler, CPU hotplug lock is held, and blk-mq's handler is waiting for inflight IO 2) error handler is waiting for CPU hotplug lock 3) inflight IO can't be completed in blk-mq's CPU hotplug handler because error handling can't provide forward progress. Solve the deadlock by not holding CPU hotplug lock in group_cpus_evenly(), in which two stage spreads are taken: 1) the 1st stage is over all present CPUs; 2) the end stage is over all other CPUs. Turns out the two stage spread just needs consistent 'cpu_present_mask', and remove the CPU hotplug lock by storing it into one local cache. This way doesn't change correctness, because all CPUs are still covered. Cc: Keith Busch Cc: linux-nvme@lists.infradead.org Cc: linux-block@vger.kernel.org Reported-by: Yi Zhang Reported-by: Guangwu Zhang Tested-by: Guangwu Zhang Reviewed-by: Chengming Zhou Reviewed-by: Jens Axboe Signed-off-by: Ming Lei Reviewed-by: Yury Norov --- lib/group_cpus.c | 22 ++++++++++++++++------ 1 file changed, 16 insertions(+), 6 deletions(-) diff --git a/lib/group_cpus.c b/lib/group_cpus.c index aa3f6815bb12..ee272c4cefcc 100644 --- a/lib/group_cpus.c +++ b/lib/group_cpus.c @@ -366,13 +366,25 @@ struct cpumask *group_cpus_evenly(unsigned int numgrp= s) if (!masks) goto fail_node_to_cpumask; =20 - /* Stabilize the cpumasks */ - cpus_read_lock(); build_node_to_cpumask(node_to_cpumask); =20 + /* + * Make a local cache of 'cpu_present_mask', so the two stages + * spread can observe consistent 'cpu_present_mask' without holding + * cpu hotplug lock, then we can reduce deadlock risk with cpu + * hotplug code. + * + * Here CPU hotplug may happen when reading `cpu_present_mask`, and + * we can live with the case because it only affects that hotplug + * CPU is handled in the 1st or 2nd stage, and either way is correct + * from API user viewpoint since 2-stage spread is sort of + * optimization. + */ + cpumask_copy(npresmsk, data_race(cpu_present_mask)); + /* grouping present CPUs first */ ret =3D __group_cpus_evenly(curgrp, numgrps, node_to_cpumask, - cpu_present_mask, nmsk, masks); + npresmsk, nmsk, masks); if (ret < 0) goto fail_build_affinity; nr_present =3D ret; @@ -387,15 +399,13 @@ struct cpumask *group_cpus_evenly(unsigned int numgrp= s) curgrp =3D 0; else curgrp =3D nr_present; - cpumask_andnot(npresmsk, cpu_possible_mask, cpu_present_mask); + cpumask_andnot(npresmsk, cpu_possible_mask, npresmsk); ret =3D __group_cpus_evenly(curgrp, numgrps, node_to_cpumask, npresmsk, nmsk, masks); if (ret >=3D 0) nr_others =3D ret; =20 fail_build_affinity: - cpus_read_unlock(); - if (ret >=3D 0) WARN_ON(nr_present + nr_others < numgrps); =20 --=20 2.41.0