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Message-ID: <Y9RLpYGmzW1KPksE@boqun-archlinux> Date: Fri, 27 Jan 2023 14:09:41 -0800 From: Boqun Feng <boqun.feng@...il.com> To: Peter Zijlstra <peterz@...radead.org> Cc: Jules Maselbas <jmaselbas@...ray.eu>, Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>, Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org, Alan Stern <stern@...land.harvard.edu>, Andrea Parri <parri.andrea@...il.com>, Nicholas Piggin <npiggin@...il.com>, David Howells <dhowells@...hat.com>, Jade Alglave <j.alglave@....ac.uk>, Luc Maranget <luc.maranget@...ia.fr>, "Paul E. McKenney" <paulmck@...nel.org>, Akira Yokosawa <akiyks@...il.com>, Daniel Lustig <dlustig@...dia.com>, Joel Fernandes <joel@...lfernandes.org>, Jonas Oberhauser <jonas.oberhauser@...weicloud.com>, Hernan Ponce de Leon <hernan.poncedeleon@...weicloud.com>, Paul Heidekrüger <paul.heidekrueger@...tum.de>, Marco Elver <elver@...gle.com>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com> Subject: Re: [PATCH] locking/atomic: atomic: Use arch_atomic_{read,set} in generic atomic ops On Fri, Jan 27, 2023 at 03:34:33PM +0100, Peter Zijlstra wrote: > > I also noticed that GCC has some builtin/extension to do such things, > > __atomic_OP_fetch and __atomic_fetch_OP, but I do not know if this > > can be used in the kernel. > > On a per-architecture basis only, the C/C++ memory model does not match > the Linux Kernel memory model so using the compiler to generate the > atomic ops is somewhat tricky and needs architecture audits. Hijack this thread a little bit, but while we are at it, do you think it makes sense that we have a config option that allows archs to implement LKMM atomics via C11 (volatile) atomics? I know there are gaps between two memory models, but the option is only for fallback/generic implementation so we can put extra barriers/orderings to make things guaranteed to work. It'll be a code version of this document: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p0124r7.html (although I realise there may be a few mistakes in that doc since I wasn't familiar with C11 memory model when I wrote part of the doc, but these can be fixed) Another reason I ask is that since Rust is coming, we need to provide our LKMM atomics in Rust so that C code and Rust code can talk via same atomic variables, since both sides need to use the same memory model. My choices are: 1. Using FFI to call Linux atomic APIs: not inline therefore not efficient. 2. Implementing Rust LKMM atomics in asm: much more work although I'm OK if we have to do it. 3. Implementing Rust LKMM atomics with standard atomics (i.e. C/C++ atomics): * Requires Rust has "volatile" atomics, which is WIP but looks promising * Less efficient compared to choice #2 but more efficient compared to choice #1 Ideally, choice #2 is the best option for all architectures, however, if we have the generic implementation based on choice #3, for some archs it may be good enough. Thoughts? [Cc LKMM and Rust people] Regards, Boqun
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