linux-kernel - Re: [PATCH] rust/dma: Take &mut self in CoherentAllocation::field

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Message-Id: <DDUA9WI1KASO.ECSJNA3RRQEI@kernel.org>
Date: Tue, 28 Oct 2025 22:46:35 +0100
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Lyude Paul" <lyude@...hat.com>
Cc: <dri-devel@...ts.freedesktop.org>, <rust-for-linux@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, "Abdiel Janulgue"
 <abdiel.janulgue@...il.com>, "Daniel Almeida"
 <daniel.almeida@...labora.com>, "Robin Murphy" <robin.murphy@....com>,
 "Andreas Hindborg" <a.hindborg@...nel.org>, "Miguel Ojeda"
 <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, "Boqun Feng"
 <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Benno Lossin"
 <lossin@...nel.org>, "Alice Ryhl" <aliceryhl@...gle.com>, "Trevor Gross"
 <tmgross@...ch.edu>
Subject: Re: [PATCH] rust/dma: Take &mut self in
 CoherentAllocation::field_write()

On Tue Oct 28, 2025 at 10:18 PM CET, Lyude Paul wrote:
> At the moment - CoherentAllocation::field_write() only takes an immutable
> reference to self. This means it's possible for a user to mistakenly call
> field_write() while Rust still has a slice taken out for the coherent
> allocation:
>
>   let alloc: CoherentAllocation<CoolStruct> = /* … */;
>
>   let evil_slice = unsafe { alloc.as_slice(/* … */)? };
>   dma_write!(alloc[1].cool_field = 42); /* UB! */
>
> Keep in mind: the above example is technically a violation of the safety
> contract of as_slice(), so luckily this detail shouldn't currently be
> causing any UB in the kernel. But, there's no reason we should be solely
> relying on the safety contract for enforcing this when we can just use a
> mutable reference and already do so in other parts of the API.

While I generally agree with this, the catch is that it would also enforce that
you would need a lock for calling dma_write!() in a concurrent context.

I.e. if your CoherentAllocation is shared between tasks we can currently have
the tasks calling dma_write!() and dma_read!() concurrently without requiring a
lock for the CoherentAllocation.

Requiring a spinlock for such a case wouldn't be the end of the world of course,
but it would still be unnecessary.