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To: Peter Zijlstra CC: Jihan LIN , , , , , , , , , , , , , , , , , , References: <20260917043339.2093426-8-suleiman@google.com> <887eff66-35f3-4703-8ff7-c0c959621b69@gmail.com> <8abf8b0f-38eb-4a25-a40a-57d5439e52e7@amd.com> <20260917153649.GK4121339@noisy.programming.kicks-ass.net> Content-Language: en-US From: K Prateek Nayak In-Reply-To: <20260917153649.GK4121339@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CH1PEPF0000AD78:EE_|SN7PR12MB6958:EE_ X-MS-Office365-Filtering-Correlation-Id: 252fa664-0a68-4dbd-dc7c-08df179bf617 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|82310400026|7416014|23010399003|376014|1800799024|36860700016|13003099007|18002099003|22082099003|56012099006|11063799006|4143699003|10067099003; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:165.204.84.17;CTRY:US;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:satlexmb07.amd.com;PTR:InfoDomainNonexistent;CAT:NONE;SFS:(13230040)(82310400026)(7416014)(23010399003)(376014)(1800799024)(36860700016)(13003099007)(18002099003)(22082099003)(56012099006)(11063799006)(4143699003)(10067099003);DIR:OUT;SFP:1101; X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: MmTqQj+8OkGPqxZAauuQq7kQzQpSL1iKGlGHyxX7Znz0rj3dcudbNGISStPU4nmz42tSp7xLX6mLvGrnQxJ7jO2S3WJ7+MtXa/eXvRwDvPibMOX7YYtVshGtaDQAPyg2iOJfFrimZB0tQBejr6sbqSWyKqN05FY0DsdBHCBu776r/+HR14lOPSIR8flY1yoUXls60WZrjofp/yHW5LlW9jzBv2MU72S1zbxLVtevX5bY2/PPBflEX4xzQdnmqQIyLjnHZ1VmhnSFWCpxwFItxkR/+ApYmx2J12/HQbcekkMuUDGaUPzssmklaN+ivZ/K3DEMKKuZ+/pMxEifqKwQtIng4QSarrcYXFSRt1d0xuNrHCJ1S21v6sMNQN5krLRP48fP2eXxb7qEmQMZjMadOd/5+6aFV+6WsrKu1yfsTSuKPsef6DFiUNzBog6BNl7C X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 21 Sep 2026 04:51:12.5482 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 252fa664-0a68-4dbd-dc7c-08df179bf617 X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[satlexmb07.amd.com] X-MS-Exchange-CrossTenant-AuthSource: CH1PEPF0000AD78.namprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: SN7PR12MB6958 On 9/17/2026 9:06 PM, Peter Zijlstra wrote: >>> Could we handle cycles in find_proxy_task(), or add a chain walk for >>> deadlock detection for FUTEX_LOCK_PING like rtmutex? >> >> https://lore.kernel.org/lkml/20260714152220.4046736-1-soolaugust@gmail.com/ > > Ah yes, that thing. I would suggest to still have a hard-coded limit, > but perhaps in addition to the sequence mark. > > Without a hard-coded limit, userspace is free to create chains of > arbitrary length. This should be discouraged :-) > > Also, we need to be able to return -EDEADLK to userspace. > > Ideally userspace gets to have an extra graph walk on block though, and > not rely on pick time sanity checks. Ack! One way to do that is by setting the "p->blocked_on" to -1 when find_proxy_task() detects a chain and then clear the "p->is_blocked" making it runnable. Once the task exits out of schedule_preempt_disabled() and grabs the wait_lock, we can check the "p->blocked_on" to return -EDEADLK / stop proxy and fully block the task. Would that be acceptable? -- Thanks and Regards, Prateek