linux-kernel - Re: [PATCH 5/5] rust: fs: use READ_ONCE instead of read

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Message-ID: <aXAiJPPrlzNld3Mu@tardis-2.local>
Date: Wed, 21 Jan 2026 08:47:32 +0800
From: Boqun Feng <boqun.feng@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Will Deacon <will@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
	Richard Henderson <richard.henderson@...aro.org>,
	Matt Turner <mattst88@...il.com>,	Magnus Lindholm <linmag7@...il.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Miguel Ojeda <ojeda@...nel.org>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Trevor Gross <tmgross@...ch.edu>,	Danilo Krummrich <dakr@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	FUJITA Tomonori <fujita.tomonori@...il.com>,
	Frederic Weisbecker <frederic@...nel.org>,	Lyude Paul <lyude@...hat.com>,
 Thomas Gleixner <tglx@...utronix.de>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	John Stultz <jstultz@...gle.com>, Stephen Boyd <sboyd@...nel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,	rust-for-linux@...r.kernel.org,
 linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 5/5] rust: fs: use READ_ONCE instead of read_volatile

On Wed, Dec 31, 2025 at 12:22:29PM +0000, Alice Ryhl wrote:
> Using `READ_ONCE` is the correct way to read the `f_flags` field.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  rust/kernel/fs/file.rs | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/rust/kernel/fs/file.rs b/rust/kernel/fs/file.rs
> index 23ee689bd2400565223181645157d832a836589f..6b07f08e7012f512e53743266096ce0076d29e1c 100644
> --- a/rust/kernel/fs/file.rs
> +++ b/rust/kernel/fs/file.rs
> @@ -335,12 +335,8 @@ pub fn cred(&self) -> &Credential {
>      /// The flags are a combination of the constants in [`flags`].
>      #[inline]
>      pub fn flags(&self) -> u32 {
> -        // This `read_volatile` is intended to correspond to a READ_ONCE call.
> -        //
> -        // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount.
> -        //
> -        // FIXME(read_once): Replace with `read_once` when available on the Rust side.
> -        unsafe { core::ptr::addr_of!((*self.as_ptr()).f_flags).read_volatile() }
> +        // SAFETY: The `f_flags` field of `struct file` is readable with `READ_ONCE`.
> +        unsafe { kernel::sync::READ_ONCE(&raw const (*self.as_ptr()).f_flags) }

Not a question directly to this patch, but for FS folks: I see we read
and write `f_flags` normally (i.e. without *_ONCE() or any atomic), and
I don't see any synchronization between these read and write (maybe I'm
missing something?), if read and write can happen at the same time, it's
data race. So I assume we must have some assumption on the atomicity of
these accesses to `f_flags`, could you may share or confirm this? Thanks

Regards,
Boqun

>      }
>  }
>  
> 
> -- 
> 2.52.0.351.gbe84eed79e-goog
>