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

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Message-ID: <20250226173534.44b42190@gandalf.local.home>
Date: Wed, 26 Feb 2025 17:35:34 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Linus Torvalds <torvalds@...ux-foundation.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 14:22:26 -0800
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> > But if I used:
> >
> >         if (global > 1000)
> >                 goto out;
> >         x = global;  
> 
> which can have the TUCTOU issue because 'global' is read twice.

Correct, but if the variable had some other protection, like a lock held
when this function was called, it is fine to do and the compiler may
optimize it or not and still have the same result.

I guess you can sum this up to:

  The compiler should never assume it's safe to read a global more than the
  code specifies, but if the code reads a global more than once, it's fine
  to cache the multiple reads.

Same for writes, but I find WRITE_ONCE() used less often than READ_ONCE().
And when I do use it, it is more to prevent write tearing as you mentioned.

-- Steve