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charset=UTF-8 Date: Wed, 29 Jul 2026 23:12:04 +0900 Message-Id: Cc: "Boqun Feng" , "Gary Guo" , "Lyude Paul" , "Daniel Almeida" , =?utf-8?q?Onur_=C3=96zkan?= , "Greg Kroah-Hartman" , "Carlos Llamas" , "Luis Chamberlain" , "Petr Pavlu" , "Daniel Gomez" , "Sami Tolvanen" , "Aaron Tomlin" , "Miguel Ojeda" , =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= , "Benno Lossin" , "Andreas Hindborg" , "Trevor Gross" , "Danilo Krummrich" , "Tamir Duberstein" , , , Subject: Re: [PATCH 0/3] rust: sync: add SetOnce::try_get_or_populate() From: "Alexandre Courbot" To: "Alice Ryhl" References: <20260722-setonce-populate-v1-0-fa7455c26c42@google.com> In-Reply-To: <20260722-setonce-populate-v1-0-fa7455c26c42@google.com> X-ClientProxiedBy: TY4P301CA0094.JPNP301.PROD.OUTLOOK.COM (2603:1096:405:37a::13) To CH2PR12MB3990.namprd12.prod.outlook.com (2603:10b6:610:28::18) Precedence: bulk X-Mailing-List: linux-modules@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CH2PR12MB3990:EE_|LV1PR12MB999305:EE_ X-MS-Office365-Filtering-Correlation-Id: c6833d7c-b38d-485a-2f1d-08deed7b604d X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|10070799003|1800799024|366016|23010399003|22082099003|18002099003|6133799003|11063799006|56012099006|10067099003; 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This means > that naive loops using as_ref() and populate() can lead to spinning on > the initialization, which is best avoided. Thus, provide a helper that > avoids this issue using a user-provided lock. > > One potential alternative is to change populate() so that the failing > caller actually does synchronize with the successful call to populate(). > However, this is somewhat tricky: > > * There are users of SetOnce that construct it in const context, and we > currently don't have the ability to do that for most locks, so we > cannot easily add a lock to SetOnce. > * Just spinning on the atomic is undesirable unless we disable > preemption in the success path. If we do disable preemption, then that > raises complications for handling the PREEMPT_RT case. > * It also raises questions about deadlocks if populate() is called from > irqs. > > By using a user-provided lock, we do not have to worry about these > issues inside SetOnce. > > This series is based on char-misc-next. > > Signed-off-by: Alice Ryhl If would be nice if the series could also include a patch updating the GEM shmem module, which is another potential user of `try_get_or_populate` - in particular since the current implementation doesn't seem to be a perfect match yet (see my comments on patch 2) so it would be an opportunity to refine it.