linux-kernel - Re: C aggregate passing (Rust kernel policy)

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Message-ID: <CAHk-=whsAoXTR+7=SxHykUs3zYzjSkY8Up2MDpjJF_PK8yVYQg@mail.gmail.com>
Date: Wed, 26 Feb 2025 16:04:19 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Martin Uecker <uecker@...raz.at>, Ralf Jung <post@...fj.de>, 
	"Paul E. McKenney" <paulmck@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, 
	Ventura Jack <venturajack85@...il.com>, Kent Overstreet <kent.overstreet@...ux.dev>, 
	Gary Guo <gary@...yguo.net>, airlied@...il.com, boqun.feng@...il.com, 
	david.laight.linux@...il.com, ej@...i.de, gregkh@...uxfoundation.org, 
	hch@...radead.org, hpa@...or.com, ksummit@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, miguel.ojeda.sandonis@...il.com, 
	rust-for-linux@...r.kernel.org
Subject: Re: C aggregate passing (Rust kernel policy)

On Wed, 26 Feb 2025 at 15:27, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> My point is that the compiler is free to turn that into:
>
>         r = READ_ONCE(global);
>         if (r > 1000)
>                 goto out;
>         x = r;
>
> and not change the expected result.

Yes.

It is safe to *combine* reads - it's what the CPU will effectively do
anyway (modulo MMIO, which as mentioned is why volatile is so special
and so different).

It's just not safe to split them.

        Linus