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[103.168.172.201]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-4a7fc1061d3sm12664251cf.15.2025.06.27.06.53.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 27 Jun 2025 06:53:36 -0700 (PDT) Received: from phl-compute-02.internal (phl-compute-02.phl.internal [10.202.2.42]) by mailfauth.phl.internal (Postfix) with ESMTP id 52B84F40066; Fri, 27 Jun 2025 09:53:35 -0400 (EDT) Received: from phl-mailfrontend-01 ([10.202.2.162]) by phl-compute-02.internal (MEProxy); Fri, 27 Jun 2025 09:53:35 -0400 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgeeffedrtdefgdefvdefucetufdoteggodetrfdotf fvucfrrhhofhhilhgvmecuhfgrshhtofgrihhlpdfurfetoffkrfgpnffqhgenuceurghi lhhouhhtmecufedttdenucesvcftvggtihhpihgvnhhtshculddquddttddmnecujfgurh epfffhvfevuffkfhggtggujgesthdtredttddtvdenucfhrhhomhepuehoqhhunhcuhfgv nhhguceosghoqhhunhdrfhgvnhhgsehgmhgrihhlrdgtohhmqeenucggtffrrghtthgvrh hnpedugeetueejudegieffvdevffektefgvdfhgfekvdduffeuieduueekkeetleevleen ucffohhmrghinheprhgvrgguhidrshhtohhrvgdprhgvrgguhidrtghomhhprghrvgenuc evlhhushhtvghrufhiiigvpedtnecurfgrrhgrmhepmhgrihhlfhhrohhmpegsohhquhhn odhmvghsmhhtphgruhhthhhpvghrshhonhgrlhhithihqdeiledvgeehtdeigedqudejje ekheehhedvqdgsohhquhhnrdhfvghngheppehgmhgrihhlrdgtohhmsehfihigmhgvrdhn rghmvgdpnhgspghrtghpthhtohepvdeipdhmohguvgepshhmthhpohhuthdprhgtphhtth hopehlohhsshhinheskhgvrhhnvghlrdhorhhgpdhrtghpthhtoheplhhinhhugidqkhgv rhhnvghlsehvghgvrhdrkhgvrhhnvghlrdhorhhgpdhrtghpthhtoheprhhushhtqdhfoh hrqdhlihhnuhigsehvghgvrhdrkhgvrhhnvghlrdhorhhgpdhrtghpthhtoheplhhkmhhm sehlihhsthhsrdhlihhnuhigrdguvghvpdhrtghpthhtoheplhhinhhugidqrghrtghhse hvghgvrhdrkhgvrhhnvghlrdhorhhgpdhrtghpthhtohepohhjvggurgeskhgvrhhnvghl rdhorhhgpdhrtghpthhtoheprghlvgigrdhgrgihnhhorhesghhmrghilhdrtghomhdprh gtphhtthhopehgrghrhiesghgrrhihghhuohdrnhgvthdprhgtphhtthhopegsjhhorhhn fegpghhhsehprhhothhonhhmrghilhdrtghomh X-ME-Proxy: Feedback-ID: iad51458e:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Fri, 27 Jun 2025 09:53:33 -0400 (EDT) Date: Fri, 27 Jun 2025 06:53:32 -0700 From: Boqun Feng To: Benno Lossin Cc: linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, lkmm@lists.linux.dev, linux-arch@vger.kernel.org, Miguel Ojeda , Alex Gaynor , Gary Guo , =?iso-8859-1?Q?Bj=F6rn?= Roy Baron , Andreas Hindborg , Alice Ryhl , Trevor Gross , Danilo Krummrich , Will Deacon , Peter Zijlstra , Mark Rutland , Wedson Almeida Filho , Viresh Kumar , Lyude Paul , Ingo Molnar , Mitchell Levy , "Paul E. McKenney" , Greg Kroah-Hartman , Linus Torvalds , Thomas Gleixner Subject: Re: [PATCH v5 05/10] rust: sync: atomic: Add atomic {cmp,}xchg operations Message-ID: References: <20250618164934.19817-1-boqun.feng@gmail.com> <20250618164934.19817-6-boqun.feng@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Fri, Jun 27, 2025 at 10:58:43AM +0200, Benno Lossin wrote: > On Wed Jun 18, 2025 at 6:49 PM CEST, Boqun Feng wrote: > > +impl Atomic > > +where > > + T::Repr: AtomicHasXchgOps, > > +{ > > + /// Atomic exchange. > > + /// > > + /// # Examples > > + /// > > + /// ```rust > > + /// use kernel::sync::atomic::{Atomic, Acquire, Relaxed}; > > + /// > > + /// let x = Atomic::new(42); > > + /// > > + /// assert_eq!(42, x.xchg(52, Acquire)); > > + /// assert_eq!(52, x.load(Relaxed)); > > + /// ``` > > + #[doc(alias("atomic_xchg", "atomic64_xchg"))] > > + #[inline(always)] > > + pub fn xchg(&self, v: T, _: Ordering) -> T { > > Can we name this `exchange`? > FYI, in Rust std, this operation is called `swap()`, what's the reason of using a name that is neither the Rust convention nor Linux kernel convention? As for naming, the reason I choose xchg() and cmpxchg() is because they are the name LKMM uses for a long time, to use another name, we have to have a very good reason to do so and I don't see a good reason that the other names are better, especially, in our memory model, we use xchg() and cmpxchg() a lot, and they are different than Rust version where you can specify orderings separately. Naming LKMM xchg()/cmpxchg() would cause more confusion I believe. Same answer for compare_exchange() vs cmpxchg(). > > + let v = T::into_repr(v); > > + let a = self.as_ptr().cast::(); > > + > > + // SAFETY: > > + // - For calling the atomic_xchg*() function: > > + // - `self.as_ptr()` is a valid pointer, and per the safety requirement of `AllocAtomic`, > > + // a `*mut T` is a valid `*mut T::Repr`. Therefore `a` is a valid pointer, > > + // - per the type invariants, the following atomic operation won't cause data races. > > + // - For extra safety requirement of usage on pointers returned by `self.as_ptr(): > > + // - atomic operations are used here. > > + let ret = unsafe { > > + match Ordering::TYPE { > > + OrderingType::Full => T::Repr::atomic_xchg(a, v), > > + OrderingType::Acquire => T::Repr::atomic_xchg_acquire(a, v), > > + OrderingType::Release => T::Repr::atomic_xchg_release(a, v), > > + OrderingType::Relaxed => T::Repr::atomic_xchg_relaxed(a, v), > > + } > > + }; > > + > > + T::from_repr(ret) > > + } > > + > > + /// Atomic compare and exchange. > > + /// > > + /// Compare: The comparison is done via the byte level comparison between the atomic variables > > + /// with the `old` value. > > + /// > > + /// Ordering: When succeeds, provides the corresponding ordering as the `Ordering` type > > + /// parameter indicates, and a failed one doesn't provide any ordering, the read part of a > > + /// failed cmpxchg should be treated as a relaxed read. > > This is a bit confusing to me. The operation has a store and a load > operation and both can have different orderings (at least in Rust > userland) depending on the success/failure of the operation. In > userland, I can supply `AcqRel` and `Acquire` to ensure that I always > have Acquire semantics on any read and `Release` semantics on any write > (which I would think is a common case). How do I do this using your API? > Usually in kernel that means in a failure case you need to use a barrier afterwards, for example: if (old != cmpxchg(v, old, new)) { smp_mb(); // ^ following memory operations are ordered against. } > Don't I need `Acquire` semantics on the read in order for > `compare_exchange` to give me the correct behavior in this example: > > pub struct Foo { > data: Atomic, > new: Atomic, > ready: Atomic, > } > > impl Foo { > pub fn new() -> Self { > Self { > data: Atomic::new(0), > new: Atomic::new(false), > ready: Atomic::new(false), > } > } > > pub fn get(&self) -> Option { > if self.new.compare_exchange(true, false, Release).is_ok() { You should use `Full` if you want AcqRel-like behavior when succeed. > let val = self.data.load(Acquire); > self.ready.store(false, Release); > Some(val) > } else { > None > } > } > > pub fn set(&self, val: u64) -> Result<(), u64> { > if self.ready.compare_exchange(false, true, Release).is_ok() { Same. Regards, Boqun > self.data.store(val, Release); > self.new.store(true, Release); > } else { > Err(val) > } > } > } > > IIUC, you need `Acquire` ordering on both `compare_exchange` operations' > reads for this to work, right? Because if they are relaxed, this could > happen: > > Thread 0 | Thread 1 > ------------------------------------------------|------------------------------------------------ > get() { | set(42) { > | if ready.cmpxchg(false, true, Rel).is_ok() { > | data.store(42, Rel) > | new.store(true, Rel) > if new.cmpxchg(true, false, Rel).is_ok() { | > let val = self.data.load(Acq); // reads 0 | > ready.store(false, Rel); | > Some(val) | > } | } > } | } > > So essentially, the `data.store` operation is not synchronized, because > the read on `new` is not `Acquire`. > [...]