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Message-ID: <272fb0fa-bff7-4ccf-bea1-fba388c5d512@paulmck-laptop>
Date: Thu, 26 Oct 2023 10:08:27 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Gary Guo <gary@...yguo.net>, Boqun Feng <boqun.feng@...il.com>,
rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-arch@...r.kernel.org, llvm@...ts.linux.dev,
Miguel Ojeda <ojeda@...nel.org>,
Alex Gaynor <alex.gaynor@...il.com>,
Wedson Almeida Filho <wedsonaf@...il.com>,
Björn Roy Baron <bjorn3_gh@...tonmail.com>,
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Alice Ryhl <aliceryhl@...gle.com>,
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Jade Alglave <j.alglave@....ac.uk>,
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linux-fsdevel@...r.kernel.org,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC] rust: types: Add read_once and write_once
On Thu, Oct 26, 2023 at 01:16:25PM +0200, Peter Zijlstra wrote:
> On Thu, Oct 26, 2023 at 11:36:10AM +0100, Gary Guo wrote:
>
> > There's two reasons that we are using volatile read/write as opposed to
> > relaxed atomic:
> > * Rust lacks volatile atomics at the moment. Non-volatile atomics are
> > not sufficient because the compiler is allowed (although they
> > currently don't) optimise atomics. If you have two adjacent relaxed
> > loads, they could be merged into one.
>
> Ah yes, that would be problematic, eg, if lifted out of a loop things
> could go sideways fast.
>
> > * Atomics only works for integer types determined by the platform. On
> > some 32-bit platforms you wouldn't be able to use 64-bit atomics at
> > all, and on x86 you get less optimal sequence since volatile load is
> > permitted to tear while atomic load needs to use LOCK CMPXCHG8B.
>
> We only grudgingly allowed u64 READ_ONCE() on 32bit platforms because
> the fallout was too numerous to fix. Some of them are probably bugs.
>
> Also, I think cmpxchg8b without lock prefix would be sufficient, but
> I've got too much of a head-ache to be sure. Worse is that we still
> support targets without cmpxchg8b.
Plus cmpxchg8b can be quite a bit heavier weight than READ_ONCE(),
in some cases to the point that you would instead use some other
concurrency design.
> It might be interesting to make the Rust side more strict in this regard
> and see where/when we run into trouble.
And maybe have some other name for READ_ONCE() that is permitted to tear.
> > * Atomics doesn't work for complex structs. Although I am not quite sure
> > of the value of supporting it.
>
> So on the C side we mandate the size is no larger than machine word,
> with the exception of the u64 on 32bit thing. We don't mandate strict
> integer types because things like pte_t are wrapper types.
On C-language atomics, people who have talked about implementing atomics
for objects too large for tear-free loads and stores have tended to want
ot invent locks. :-(
Thanx, Paul
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