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From: Boqun Feng <boqun.feng@gmail.com>
To: Gary Guo <gary@garyguo.net>
Cc: "John Hubbard" <jhubbard@nvidia.com>,
	"Miguel Ojeda" <miguel.ojeda.sandonis@gmail.com>,
	rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-arch@vger.kernel.org, llvm@lists.linux.dev,
	"Miguel Ojeda" <ojeda@kernel.org>,
	"Alex Gaynor" <alex.gaynor@gmail.com>,
	"Wedson Almeida Filho" <wedsonaf@gmail.com>,
	"Björn Roy Baron" <bjorn3_gh@protonmail.com>,
	"Benno Lossin" <benno.lossin@proton.me>,
	"Andreas Hindborg" <a.hindborg@samsung.com>,
	"Alice Ryhl" <aliceryhl@google.com>,
	"Alan Stern" <stern@rowland.harvard.edu>,
	"Andrea Parri" <parri.andrea@gmail.com>,
	"Will Deacon" <will@kernel.org>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Nicholas Piggin" <npiggin@gmail.com>,
	"David Howells" <dhowells@redhat.com>,
	"Jade Alglave" <j.alglave@ucl.ac.uk>,
	"Luc Maranget" <luc.maranget@inria.fr>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	"Akira Yokosawa" <akiyks@gmail.com>,
	"Daniel Lustig" <dlustig@nvidia.com>,
	"Joel Fernandes" <joel@joelfernandes.org>,
	"Nathan Chancellor" <nathan@kernel.org>,
	"Nick Desaulniers" <ndesaulniers@google.com>,
	kent.overstreet@gmail.com,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	elver@google.com, "Mark Rutland" <mark.rutland@arm.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	torvalds@linux-foundation.org,
	linux-arm-kernel@lists.infradead.org,
	linux-fsdevel@vger.kernel.org, "Trevor Gross" <tmgross@umich.edu>,
	dakr@redhat.com
Subject: Re: [RFC 2/2] rust: sync: Add atomic support
Date: Sun, 16 Jun 2024 22:42:19 -0700	[thread overview]
Message-ID: <Zm_Mu7C76jpMyRy6@Boquns-Mac-mini.home> (raw)
In-Reply-To: <Zm_LTXm3wJhcQIwI@Boquns-Mac-mini.home>

On Sun, Jun 16, 2024 at 10:36:13PM -0700, Boqun Feng wrote:
> On Sun, Jun 16, 2024 at 08:06:05AM -0700, Boqun Feng wrote:
> [...]
> > > 
> > > Note that crossbeam's AtomicCell is also generic, and crossbeam is used
> > > by tons of crates. As Miguel mentioned, I think it's very likely that in
> > > the future we want be able to do atomics on new types (e.g. for
> > > seqlocks perhaps). We probably don't need the non-lock-free fallback of
> > 
> > Good, another design bit, thank you!
> > 
> > What's our overall idea on sub-word types, like Atomic<u8> and
> > Atomic<u16>, do we plan to say no to them, or they could have a limited
> > APIs? IIUC, some operations on them are relatively sub-optimal on some
> > architectures, supporting the same set of API as i32 and i64 is probably
> > a bad idea.
> > 
> > Another thing in my mind is making `Atomic<T>`
> > 
> > 	pub struct Atomic<T: Send + ...> { ... }
> > 
> > so that `Atomic<T>` will always be `Sync`, because quite frankly, an
> > atomic type that cannot `Sync` is pointless.
> > 
> 
> Also, how do we avoid this issue [1] in kernel?
> 
> `atomic_load()` in C is implemented as READ_ONCE() and it's, at most
> time, a volatile read, so the eventual code is:
> 
>     let a: (u8, u16) = (1, 2);
>     let b = unsafe { core::ptr::read_volatile::<i32>(&a as *const _ as *const i32) };
> 

^^^^ this line should really be:

	let b: (u8, u16) = unsafe { transmute_copy(&read_volatile::<i32>(&a as *const _ as *const i32)) };

but you get the idea.

Regards,
Boqun

> I know we probably ignore data race here and treat `read_volatile` as a
> dependency read per LKMM [2]. But this is an using of uninitialized
> data, so it's a bit different.
> 
> We can do what https://crates.io/crates/atomic does:
> 
> 	pub struct Atomic<T: NoUninit + ..> { ... }
> 
> , where `NoUinit` means no internal padding bytes, but it loses the
> ability to put a 
> 
> 	#[repr(u32)]
> 	pub enum Foo { .. }
> 
> into `Atomic<T>`, right? Which is probably a case you want to support?
> 
> Regards,
> Boqun
> 
> [1]: https://github.com/crossbeam-rs/crossbeam/issues/748#issuecomment-1133926617
> [2]: tools/memory-model/Documentation/access-marking.txt
> 
> > Regards,
> > Boqun
> > 
> > > crossbeam's AtomicCell, but the lock-free subset with newtype support
> > > is desirable.
> > > 
> > > People in general don't use the `atomic` crate because it provides no
> > > additional feature compared to the standard library. But it doesn't
> > > really mean that the standard library's atomic design is good.
> > > 
> > > People decided to use AtomicT and NonZeroT instead of Atomic<T> or
> > > NonZero<T> long time ago, but many now thinks the decision was bad.
> > > Introduction of NonZero<T> is a good example of it. NonZeroT are now
> > > all type aliases of NonZero<T>.
> > > 
> > > I also don't see any downside in using generics. We can provide type
> > > aliases so people can use `AtomicI32` and `AtomicI64` when they want
> > > their code to be compatible with userspace Rust can still do so.
> > > 
> > > `Atomic<i32>` is also just aesthetically better than `AtomicI32` IMO.
> > > When all other types look like `NonZero<i32>`, `Wrapping<i32>`, I don't
> > > think we should have `AtomicI32` just because "it's done this way in
> > > Rust std". Our alloc already deviates a lot from Rust std.
> > > 
> > > Best,
> > > Gary

  reply	other threads:[~2024-06-17  5:42 UTC|newest]

Thread overview: 56+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-06-12 22:30 [RFC 0/2] Initial LKMM atomics support in Rust Boqun Feng
2024-06-12 22:30 ` [RFC 1/2] rust: Introduce atomic API helpers Boqun Feng
2024-06-13  5:38   ` Greg Kroah-Hartman
2024-06-13  9:17     ` Peter Zijlstra
2024-06-13 10:03       ` Greg Kroah-Hartman
2024-06-13 10:36       ` Mark Rutland
2024-06-14 10:31   ` Mark Rutland
2024-06-14 20:13     ` Boqun Feng
2024-06-12 22:30 ` [RFC 2/2] rust: sync: Add atomic support Boqun Feng
2024-06-13  5:40   ` Greg Kroah-Hartman
2024-06-13 13:44   ` Gary Guo
2024-06-13 16:30     ` Boqun Feng
2024-06-13 17:19       ` Gary Guo
2024-06-13 17:22       ` Miguel Ojeda
2024-06-13 19:05         ` Boqun Feng
2024-06-14  9:59           ` Miguel Ojeda
2024-06-14 14:33             ` Boqun Feng
2024-06-14 21:22               ` Benno Lossin
2024-06-15  1:33                 ` Boqun Feng
2024-06-15  7:09                   ` Benno Lossin
2024-06-15 22:12                     ` Boqun Feng
2024-06-16  9:46                       ` Benno Lossin
2024-06-16 14:08                         ` Boqun Feng
2024-06-16 15:06                           ` Benno Lossin
2024-06-16 15:34                             ` Boqun Feng
2024-06-16 15:55                               ` Benno Lossin
2024-06-16 16:30                                 ` Boqun Feng
2024-06-19  9:09                                   ` Benno Lossin
2024-06-19 15:00                                     ` Boqun Feng
2024-06-16 17:05                         ` Boqun Feng
2024-06-16  9:51                       ` Kent Overstreet
2024-06-16 14:16                         ` Boqun Feng
2024-06-16 14:35                           ` Boqun Feng
2024-06-16 15:14                           ` Miguel Ojeda
2024-06-16 15:32                             ` Kent Overstreet
2024-06-16 15:54                               ` Boqun Feng
2024-06-16 17:30                                 ` Boqun Feng
2024-06-16 17:59                                   ` Kent Overstreet
2024-06-16 15:50                             ` Boqun Feng
2024-06-16 15:23                           ` Kent Overstreet
2024-06-15  1:03             ` John Hubbard
2024-06-15  1:24               ` Boqun Feng
2024-06-15  1:28                 ` John Hubbard
2024-06-15  2:39                   ` Boqun Feng
2024-06-15  2:51                     ` John Hubbard
2024-06-16 14:51                     ` Gary Guo
2024-06-16 15:06                       ` Boqun Feng
2024-06-17  5:36                         ` Boqun Feng
2024-06-17  5:42                           ` Boqun Feng [this message]
2024-06-19  9:30                           ` Benno Lossin
2024-06-16  0:51             ` Andrew Lunn
2024-06-14  9:51       ` Peter Zijlstra
2024-06-14 14:18         ` Boqun Feng
2024-06-13 20:25   ` Boqun Feng
2024-06-14 10:40   ` Mark Rutland
2024-06-14 20:20     ` Boqun Feng

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