Received-SPF: pass (google.com: domain of linux-kernel+bounces-112184-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1;
Date: Fri, 22 Mar 2024 22:33:13 -0400
From: Kent Overstreet <kent.overstreet@linux.dev>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, 
	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>, 
	Gary Guo <gary@garyguo.net>, =?utf-8?B?QmrDtnJu?= 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>, linux-arm-kernel@lists.infradead.org, 
	linux-fsdevel@vger.kernel.org
Subject: Re: [WIP 0/3] Memory model and atomic API in Rust
Message-ID: <34r4signulvsclmsiqgghskmj5xce3zs5hwgfulzaez2wdyklr@ck6zrj732c4m>
References: <20240322233838.868874-1-boqun.feng@gmail.com>
 <s2jeqq22n5ef5jknaps37mfdjvuqrns4w7i22qp2r7r4bzjqs2@my3eyxoa3pl3>
 <CAHk-=whY5A=S=bLwCFL=043DoR0TTgSDUmfPDx2rXhkk3KANPQ@mail.gmail.com>
 <u2suttqa4c423q4ojehbucaxsm6wguqtgouj7vudp55jmuivq3@okzfgryarwnv>
 <CAHk-=whkQk=zq5XiMcaU3xj4v69+jyoP-y6Sywhq-TvxSSvfEA@mail.gmail.com>
 <3modld2dafaqjxa2b7jln47ws4ylzhbsvhvnphoklwvzange5p@wlir7276aitp>
 <Zf491DuptReGqvfd@Boquns-Mac-mini.home>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Zf491DuptReGqvfd@Boquns-Mac-mini.home>

On Fri, Mar 22, 2024 at 07:26:28PM -0700, Boqun Feng wrote:
> On Fri, Mar 22, 2024 at 10:07:31PM -0400, Kent Overstreet wrote:
> [...]
> > > Boqun already mentioned the "mixing access sizes", which is actually
> > > quite fundamental in the kernel, where we play lots of games with that
> > > (typically around locking, where you find patterns line unlock writing
> > > a zero to a single byte, even though the whole lock data structure is
> > > a word). And sometimes the access size games are very explicit (eg
> > > lib/lockref.c).
> > 
> > I don't think mixing access sizes should be a real barrier. On the read
> 
> Well, it actually is, since mixing access sizes is, guess what,
> an undefined behavior:
> 
> (example in https://doc.rust-lang.org/std/sync/atomic/#memory-model-for-atomic-accesses)
> 
> 	thread::scope(|s| {
> 	    // This is UB: using different-sized atomic accesses to the same data
> 	    s.spawn(|| atomic.store(1, Ordering::Relaxed));
> 	    s.spawn(|| unsafe {
> 		let differently_sized = transmute::<&AtomicU16, &AtomicU8>(&atomic);
> 		differently_sized.store(2, Ordering::Relaxed);
> 	    });
> 	});
> 
> Of course, you can say "I will just ignore the UB", but if you have to
> ignore "compiler rules" to make your code work, why bother use compiler
> builtin in the first place? Being UB means they are NOT guaranteed to
> work.

That's not what I'm proposing - you'd need additional compiler support.
but the new intrinsic would be no different, semantics wise for the
compiler to model, than a "lock orb".