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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 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue Sep 29, 2026 at 4:42 PM BST, Mathieu Desnoyers wrote: > On 2026-09-29 11:16, Linus Torvalds wrote: >> On Tue, 29 Sept 2026 at 08:04, Mathieu Desnoyers >> wrote: >>> >>> I agree that doing it in asm would generate better code. 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