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charset=UTF-8 Date: Tue, 16 Dec 2025 23:38:52 +0900 Message-Id: Cc: "Alice Ryhl" , "Simona Vetter" , "Bjorn Helgaas" , =?utf-8?q?Krzysztof_Wilczy=C5=84ski?= , "Miguel Ojeda" , "Boqun Feng" , "Gary Guo" , =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= , "Benno Lossin" , "Andreas Hindborg" , "Trevor Gross" , "Alistair Popple" , "Joel Fernandes" , "Eliot Courtney" , , , , , Subject: Re: [PATCH 1/7] rust: pci: pass driver data by value to `unbind` From: "Alexandre Courbot" To: "Danilo Krummrich" , "Alexandre Courbot" X-Mailer: aerc 0.21.0-0-g5549850facc2 References: <20251216-nova-unload-v1-0-6a5d823be19d@nvidia.com> <20251216-nova-unload-v1-1-6a5d823be19d@nvidia.com> In-Reply-To: X-ClientProxiedBy: TY4PR01CA0019.jpnprd01.prod.outlook.com (2603:1096:405:2bf::9) To CH2PR12MB3990.namprd12.prod.outlook.com (2603:10b6:610:28::18) Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CH2PR12MB3990:EE_|PH8PR12MB6772:EE_ X-MS-Office365-Filtering-Correlation-Id: 536096b6-3fd4-4d21-4a4e-08de3cb0d76d X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|10070799003|1800799024|376014|366016|7416014; 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This becomes a problem in e.g. Nova, which needs >> to invoke mutable methods when unbinding. >> >> `remove_callback` retrieves the driver data by value, and drops it right >> after the call to `T::unbind`, meaning it is the only reference to the >> driver data by the time `T::unbind` is called. >> >> There is thus no reason for not granting full ownership of the data to >> `T::unbind`, so do it. > > There are multiple reasons I did avoid this for: > > (1) Race conditions > > A driver can call Device::drvdata() and obtain a reference to the driver'= s > device private data as long as it has a &Device and asserts the co= rrect > type of the driver's device private data [1]. > > Assume you have an IRQ registration, for instance, that lives within this= device > private data. Within the IRQ handler, nothing prevents us from calling > Device::drvdata() given that the IRQ handler has a Device referenc= e. > > Consequently, with passing the device private data by value to unbind() i= t can > happen that we have both a mutable and immutable reference at of the devi= ce > private data at the same time. > > The same is true for a lot of other cases, such as work items or workqueu= es that > are scoped to the Device being bound living within the device private dat= a. > > More generally, you can't take full ownership of the device private data = as long > as the device is not yet fully unbound (which is not yet the case in unbi= nd()). Ah, I completely ignored the fact that we can indeed have other references to the private data! The fact that `unbind` works with `Device` should have given me a hint, but somehow I blissfully ignored that. ^_^; I will implement some basic locking on the command queue so we can work with a non-mutable reference.