From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from PH7PR06CU001.outbound.protection.outlook.com (mail-westus3azon11010000.outbound.protection.outlook.com [52.101.201.0]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C4EE1348457 for ; Tue, 10 Mar 2026 06:47:39 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=fail smtp.client-ip=52.101.201.0 ARC-Seal:i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773125260; cv=fail; b=bRxso0AA02VuYA3U5YCCc1ONACoe4hf0lZ2Yekt6xjZ0Woc1/y2mr9MKiCHZl7rgnu/ROv4ZsfQ02KprsB7BEYPvklRNb4NqlAWhpPmCHra1VEX6c1yBmY/2Akqk7uG73aKPfIbBEKzA0Dk/1f7Tl21yhkYsGCg766EW8yMJ8Q8= ARC-Message-Signature:i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773125260; c=relaxed/simple; bh=Pfuemmf9sch8ZhJu3GVqYfZt/tljVmRXZf16GlA9dA4=; h=Date:From:To:Cc:Subject:Message-ID:References:Content-Type: Content-Disposition:In-Reply-To:MIME-Version; b=p/lT1fJLbrFtbykEeazWEyJcAYY78ywLj8KD6lhXdCSOAeHVc0L/AMpjX4v/MLehLB4/TcTWmyfE+C3rSdOuG4KyY2jR72Nt7idTSgjtnknTPrEBHycMNBVXaRp7uBSp3zrErY1I0hNYfI/0hXEd2+ee+PyXZ9nYNa6lVaikou8= ARC-Authentication-Results:i=2; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=nvidia.com; spf=fail smtp.mailfrom=nvidia.com; dkim=pass (2048-bit key) header.d=Nvidia.com header.i=@Nvidia.com header.b=gkOVopUA; arc=fail smtp.client-ip=52.101.201.0 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=nvidia.com Authentication-Results: smtp.subspace.kernel.org; spf=fail smtp.mailfrom=nvidia.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=Nvidia.com header.i=@Nvidia.com header.b="gkOVopUA" ARC-Seal: i=1; a=rsa-sha256; s=arcselector10001; d=microsoft.com; cv=none; b=gwjeH2i7WnWw67KAl0EccDX6fQ9lFh4nXMvPjaWr63BD83mhxzYKbz8epWXUXbt7nqWQfQbVTqPA2JYLgIzZUqKdW1WvH3+6TIYOsR34WV9SQFUJJcBueypgx3lMQG269CqhhsvzF4AZklYeaCfm3YWDnfA1EQ1s9GtptWZTQt8rQ6Un0e6umPRkgDTEn2wDTnRhKXYcqOTJJe7bFDJccL5hiplI4Fr6U5paaRAaBbR9YkP3EZ9Ne/Pi84Nl/OleyuzrMHp2wEzEViXG/iHcWxE5T8K8Des1R8hiP4rhu4sUFGb310o7rGkNGqxOSRlYc/FLd+xvPVlojar+CJejlg== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=microsoft.com; s=arcselector10001; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-AntiSpam-MessageData-ChunkCount:X-MS-Exchange-AntiSpam-MessageData-0:X-MS-Exchange-AntiSpam-MessageData-1; bh=9bA2A8OIIDR8tWUzmWSvTO2QLjCWTGYqrdN5o1q8qLY=; b=d98oiNXobsryKqWdlWEf+XNzBhvUHXQYzzYqYLMt922DLt8hTfkDCAAtB6QVuYmX3NGf0RaQUWzy2N5WOaux6wHMLznZqaqCcgbWBKReZwMnNuWvQ80b75qaqVqmk1wCBGcwhZcQl4qPikWYKtesK925+AjY48X/OvHWTSOWWhcHU0Etuz2+EQySi9EGpttoCWb9MFBzSK+904ZLq3JM1fY7yPwWI0UuZuPx36l7l4Z5bhjTtqyWvLHvwKeOJaMAJbzV/sfe48yGV3mmBSBXDufmPxl0grCliEvAltCShNYU6ghpJZS5i2kOz1596fiJnMzqLavU8SuoYOxX4Ma/0w== ARC-Authentication-Results: i=1; mx.microsoft.com 1; spf=pass smtp.mailfrom=nvidia.com; dmarc=pass action=none header.from=nvidia.com; dkim=pass header.d=nvidia.com; arc=none DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=Nvidia.com; s=selector2; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=9bA2A8OIIDR8tWUzmWSvTO2QLjCWTGYqrdN5o1q8qLY=; b=gkOVopUAFE7bh22BAxuyvQHqZwbZTewNbVs37inPtUcYaRGqFOdmj5iSh9gaZedd8BCSFagiITlH/71Ddr+8HgiMwrL7kmqS8RKHtq+Tsf4NXlnBrHJ+wj8IWWi/qRDmyvRGzNDYqLL1xbE2Mn4N70Qc6DXtE9ZkZ2AvoFDUsXfhqbkzqIwCt4PyQyLUnULU1XiHr7jc2+DAnYFe3SeRVb82neJK7TrZ9vEPJ20daTU8A8B460GM2en/PDVSGGAxDCLPJXv9krJFD2DofQWfodlQI5l4tkC0V6gpFI3YDPq0INrAm+vNAOQvEEz29/sbEmcDuJOj7FTWLQlOabSVZQ== Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=nvidia.com; Received: from LV8PR12MB9620.namprd12.prod.outlook.com (2603:10b6:408:2a1::19) by PH0PR12MB8007.namprd12.prod.outlook.com (2603:10b6:510:28e::5) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9700.11; Tue, 10 Mar 2026 06:47:36 +0000 Received: from LV8PR12MB9620.namprd12.prod.outlook.com ([fe80::299d:f5e0:3550:1528]) by LV8PR12MB9620.namprd12.prod.outlook.com ([fe80::299d:f5e0:3550:1528%5]) with mapi id 15.20.9654.022; Tue, 10 Mar 2026 06:47:36 +0000 Date: Tue, 10 Mar 2026 07:47:26 +0100 From: Andrea Righi To: Tejun Heo Cc: David Vernet , Changwoo Min , sched-ext@lists.linux.dev, Emil Tsalapatis , Cheng-Yang Chou , linux-kernel@vger.kernel.org Subject: Re: [PATCH 4/5] sched_ext: Fix scx_sched_lock / rq lock ordering Message-ID: References: <20260310011653.2993712-1-tj@kernel.org> <20260310011653.2993712-5-tj@kernel.org> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-ClientProxiedBy: MI2P293CA0008.ITAP293.PROD.OUTLOOK.COM (2603:10a6:290:45::19) To LV8PR12MB9620.namprd12.prod.outlook.com (2603:10b6:408:2a1::19) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: LV8PR12MB9620:EE_|PH0PR12MB8007:EE_ X-MS-Office365-Filtering-Correlation-Id: 479a5114-d534-4c8e-a2bf-08de7e70e9ef X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|1800799024|366016|7053199007; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:LV8PR12MB9620.namprd12.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230040)(376014)(1800799024)(366016)(7053199007);DIR:OUT;SFP:1101; X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?uR42jZR5FuAvTT4H6FzmW0mv9n/Q70sTyU9JomscDPFV0lBNya0cmSMbRbkg?= =?us-ascii?Q?t/Ql8Fx9DGHZfbrLPuh5OerHjTes04LIBLKZJTDNMyKXEenndojT9XY/8/Pr?= =?us-ascii?Q?lkDfXAOVKPl86snQfeGqSTXasUX5vMbtLyJZ91IO5xPqWumWWCYXzd9EKkD5?= =?us-ascii?Q?gn3jJvdIxgm9DQcgpsZFGFXb8W5QlHuap4h4zPGaUn/llgQ+2S0P0qX0RKPg?= =?us-ascii?Q?XUP3brSoQsL9nVXlK8iiKE1QHOOHPabDF6rcW6NFkG73LfJdVVhlZrDXEZlS?= =?us-ascii?Q?zfU0BdzzaasEN+YZGdlVQIZ7QCSUVoFcbbCZyR9u79zav0qdKc77rnFvfVjB?= =?us-ascii?Q?+3VyZi37k9HxyEQq9qjirwjwsA2943Ea4SkQ8J5N1JKQmrGzHNYxCS/7sN15?= =?us-ascii?Q?OnmhEYJLod7gzpnQJZk3Sd6rzw7VkWhWN/03Aca4k8FE6O0J3fEVabw9V6xr?= =?us-ascii?Q?vXPAy00qAr8QbU/L3AEWqPBeb6SthokQeuMlGxoiVOw1uNnQ9cyccXn4XGQC?= =?us-ascii?Q?d1HGfm3AdHqix1xs+V6vdctk/Yx1hpcKn6dZ2gePHu1tzsi5/ShQjUhI7Oqv?= =?us-ascii?Q?K6eZE/Lk/hVDv8QiyYQAqHHaarAN8fWyOhfJuVcewh6t/2KSubM/Bd6xeMdw?= =?us-ascii?Q?NU0PO6MnlkS/bm05JlNuM9LGBUb1RlLbYt0JyihOwVE5w2yh/6OUnP+RHfzM?= =?us-ascii?Q?Fp0ESaY9AvTgiMWkI4biCoT0OJwgdLsrkLJOAuWdgQjnfKzuLRFSxOCx03sl?= =?us-ascii?Q?1sDnECF5IJ+ohl4+klIPVt5pMU10ueIyhUxpHfu/JcZajaWP4dkF7h400+Nn?= =?us-ascii?Q?rvPhDmNzgBj6CpJWbC9CKOOJPCxrwghjg7khy7qOK8naW7L17y6A5jTK+8GF?= =?us-ascii?Q?thhsVMRSy4jl1v1oh/gVBPFchSL6ibxUhu4SusmhdFyWcYgF2Q0S5QRF1j5u?= =?us-ascii?Q?3IFfoFKwYTZBqY8aNFOYV2VYzO5faiai/Y1UftBxEpCovH9diRh3X8GSt2dx?= =?us-ascii?Q?J8aG3q3dkzQFu0m/BUpHs/Jl3Io8cxqhuOxib5ORJPoJ1/mIPHW6qRuYpD7B?= =?us-ascii?Q?bzDz7BpzidZyUkFkByMWi6DtdviA6PxQTUIoJdz6aGp89r2eNUHrjUiwSM7s?= =?us-ascii?Q?2w29w3umd//4qh+CAXsP4MXTHXy/PuU/D5IKh0+sME06v7xC+ZktgRU3l2bw?= =?us-ascii?Q?Rgd8ZnwI2ju5LvnHjZTkzPbjbcOCXLfr1YBp8sFW2VHm4WM7V74sxySqy8Bx?= =?us-ascii?Q?8AxQxiVMJHsPA8Fe/+tQB46KvDXBvy6lRNkBmgeRcRaX5IAPNvqGpsZcy/vM?= =?us-ascii?Q?E4W7qGB0EcWStmWsZRjQr2JDMuUZjlSZEQV+mf3zs8X/EMbnfCoQVDEono7x?= =?us-ascii?Q?S59JXrhoEClnjPx/A7S9D4TGofnr8XKRTUti+88I/aOsRq6QDQCF4eFOJavM?= =?us-ascii?Q?tYCE0wsEvgwxTGQOHduKMBiHHM+WzrjZr9Nzi9oBpZhsIGRuX+7BXY6dm9Zt?= =?us-ascii?Q?x7BZonSt97+pFl8AR3w5RSS9vcIO3cpUxZif6/ouhrW47jS4OTFFCm866cY+?= =?us-ascii?Q?vCRB3Q5U4XX+V52jevStsBTmpfhrA8cDihwc5u1g6dlL9GnPj0UfFGxLycg6?= =?us-ascii?Q?fDNAdXDoVJpWz5xy48QyMLHT5DeB3xf0hsikK7/wWonv65UwsVA3CJeI6qA9?= =?us-ascii?Q?0Vl5kI9PKiRXu7c6vvKflQwOsiRMOLAfjiGLt3aB2AxfZi/raB0sDDk9Jv+Y?= =?us-ascii?Q?f4SE4EMoHQ=3D=3D?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: 479a5114-d534-4c8e-a2bf-08de7e70e9ef X-MS-Exchange-CrossTenant-AuthSource: LV8PR12MB9620.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 10 Mar 2026 06:47:36.1282 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: M3uwdCUuZ8ZzlSamPOgnCYHw/1OCpf/odoC7i6qfbQSjC/FctReNbhNe6ZNT6diMhjxnzAQjPNLaupfK06WQKw== X-MS-Exchange-Transport-CrossTenantHeadersStamped: PH0PR12MB8007 On Tue, Mar 10, 2026 at 07:39:10AM +0100, Andrea Righi wrote: > Hi Tejun, > > On Mon, Mar 09, 2026 at 03:16:52PM -1000, Tejun Heo wrote: > > There are two sites that nest rq lock inside scx_sched_lock: > > > > - scx_bypass() takes scx_sched_lock then rq lock per CPU to propagate > > per-cpu bypass flags and re-enqueue tasks. > > > > - sysrq_handle_sched_ext_dump() takes scx_sched_lock to iterate all > > scheds, scx_dump_state() then takes rq lock per CPU for dump. > > > > And scx_claim_exit() takes scx_sched_lock to propagate exits to > > descendants. It can be reached from scx_tick(), BPF kfuncs, and many > > other paths with rq lock already held, creating the reverse ordering: > > > > rq lock -> scx_sched_lock vs. scx_sched_lock -> rq lock > > > > Fix by flipping scx_bypass() to take rq lock first, and dropping > > scx_sched_lock from sysrq_handle_sched_ext_dump() as scx_sched_all is > > already RCU-traversable and scx_dump_lock now prevents dumping a dead > > sched. This makes the consistent ordering rq lock -> scx_sched_lock. > > > > Reported-by: Cheng-Yang Chou > > Link: http://lkml.kernel.org/r/20260309163025.2240221-1-yphbchou0911@gmail.com > > Fixes: ebeca1f930ea ("sched_ext: Introduce cgroup sub-sched support") > > Signed-off-by: Tejun Heo > > --- > > kernel/sched/ext.c | 4 +--- > > 1 file changed, 1 insertion(+), 3 deletions(-) > > > > diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c > > index cf28a8f62ad0..677c1c6c64bf 100644 > > --- a/kernel/sched/ext.c > > +++ b/kernel/sched/ext.c > > @@ -5097,8 +5097,8 @@ static void scx_bypass(struct scx_sched *sch, bool bypass) > > struct rq *rq = cpu_rq(cpu); > > struct task_struct *p, *n; > > > > - raw_spin_lock(&scx_sched_lock); > > raw_spin_rq_lock(rq); > > + raw_spin_lock(&scx_sched_lock); > > > > scx_for_each_descendant_pre(pos, sch) { > > struct scx_sched_pcpu *pcpu = per_cpu_ptr(pos->pcpu, cpu); > > @@ -7240,8 +7240,6 @@ static void sysrq_handle_sched_ext_dump(u8 key) > > struct scx_exit_info ei = { .kind = SCX_EXIT_NONE, .reason = "SysRq-D" }; > > struct scx_sched *sch; > > > > - guard(raw_spinlock_irqsave)(&scx_sched_lock); > > - > > Don't we need RCU protection here? Nevermind, __handle_sysrq() is already doing rcu_read_lock/unlock(), so this looks good. Sorry for the noise, -Andrea > > > list_for_each_entry_rcu(sch, &scx_sched_all, all) > > scx_dump_state(sch, &ei, 0, false); > > } > > Thanks, > -Andrea