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McKenney" , , "Boqun Feng" , "Joel Fernandes" , "Alan Stern" , "Greg Kroah-Hartman" , "Sebastian Andrzej Siewior" , "Will Deacon" , "Peter Zijlstra" , "John Stultz" , "Frederic Weisbecker" , "Josh Triplett" , "Uladzislau Rezki" , "Steven Rostedt" , "Lai Jiangshan" , "Zqiang" , "Ingo Molnar" , "Waiman Long" , "Mark Rutland" , "Thomas Gleixner" , "Vlastimil Babka" , , "Mateusz Guzik" , "Gary Guo" , , , , "Nikita Popov" , Subject: Re: [PATCH hazptr 2/4] compiler.h: Introduce ptr_eq() to preserve address dependency From: "Gary Guo" To: "Mathieu Desnoyers" , "Linus Torvalds" X-Mailer: aerc 0.22.0 References: <20260927155134.4740-1-mathieu.desnoyers@efficios.com> <20260927155134.4740-3-mathieu.desnoyers@efficios.com> <133ca8de-cf30-4bf5-9539-d0b73e347b22@efficios.com> <5a695b14-d25d-4c11-87f4-83c8d299d79f@efficios.com> In-Reply-To: <5a695b14-d25d-4c11-87f4-83c8d299d79f@efficios.com> X-ClientProxiedBy: LO4P123CA0620.GBRP123.PROD.OUTLOOK.COM (2603:10a6:600:294::16) To LOAP265MB8560.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:4ab::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: LOAP265MB8560:EE_|LO0P265MB3225:EE_ X-MS-Office365-Filtering-Correlation-Id: 56d70115-dc68-4d56-ba6b-08df1e45dfd6 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|10070799003|23010399003|366016|1800799024|4143699003|56012099006|10067099003|22082099003|18002099003|3023799007; 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Also, I've >>> been told that clang now preserves the address dependency in this >>> scenario, so perhaps we'd want to use preprocessor conditionals to >>> select how to implement ptr_eq based on: >>> >>> - architecture (allowing asm implementation overrides), >>> - compiler (e.g. if compiler is clang >=3D version X, just do a plain >>> comparison). Likewise for gcc if it ever decides to fix this >>> behavior. >>> >>> I think this "hide var" hack is a fallback which can be used as a >>> starting point, and then we can specialize based on architecture and >>> compiler. >>> >>> Or do you prefer this in a different order ? >>=20 >> Oh, if there's some sane way to tell that the compiler already honors >> address dependencies, then that should be done first and the whole >> thing should just become a simple >>=20 >> (a) =3D=3D (b) >>=20 >> for that situation - allowing the compiler then the freedom to do >> whatever (which can involve not using a register at all, but a compare >> to memory, or whatever - the compiler might have reasons to avoid the >> 'cmp' and use another sequence entirely) > > For the records, here is the relevant compiler discussion: > > https://github.com/llvm/llvm-project/issues/34577 > > and it seems like they still have unfixed scenarios, > which may or may not affect the hazptr use-case: > > https://github.com/llvm/llvm-project/issues/220206 > > Considering this, I would not be inclined to jump to the conclusion > that using the compiler compare for hazptr acquire load/test/reload > is safe today without a careful analysis. Out of the existing cases I think the only problematic one for hazptr is th= e comparison against costant global. Which isn't going to be a problem for us= ers that don't depend on a fixed adress. But it might be a problem for the lockdep case. If we use inline asm for ptr_eq I think there's very little optimization potentials left for compilers, so I think unconditionally using asm would b= e fine. Best, Gary