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Date: Tue, 26 Aug 2025 21:13:53 +0200
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Cc: <akpm@linux-foundation.org>, <ojeda@kernel.org>,
 <alex.gaynor@gmail.com>, <boqun.feng@gmail.com>, <gary@garyguo.net>,
 <bjorn3_gh@protonmail.com>, <lossin@kernel.org>, <a.hindborg@kernel.org>,
 <aliceryhl@google.com>, <tmgross@umich.edu>, <abdiel.janulgue@gmail.com>,
 <acourbot@nvidia.com>, <jgg@ziepe.ca>, <lyude@redhat.com>,
 <robin.murphy@arm.com>, <rust-for-linux@vger.kernel.org>,
 <linux-kernel@vger.kernel.org>
To: "Daniel Almeida" <daniel.almeida@collabora.com>
From: "Danilo Krummrich" <dakr@kernel.org>
Subject: Re: [PATCH v3 3/5] rust: scatterlist: Add abstraction for sg_table
References: <20250825132539.122412-1-dakr@kernel.org>
 <20250825132539.122412-4-dakr@kernel.org>
 <6625A8C0-33B2-4AFC-984B-130F8E06113D@collabora.com>
In-Reply-To: <6625A8C0-33B2-4AFC-984B-130F8E06113D@collabora.com>

On Tue Aug 26, 2025 at 7:40 PM CEST, Daniel Almeida wrote:
>> On 25 Aug 2025, at 10:24, Danilo Krummrich <dakr@kernel.org> wrote:
>> +// SAFETY: `SGEntry` can be accessed concurrently.
>> +unsafe impl Sync for SGEntry {}
>
> IMHO all safety comments for Sync must mention how it=E2=80=99s ok to sen=
d &T to
> another thread for a given T.

When we say "`T` can be accessed concurrently." it means that it is valid f=
or
multiple &T to be accessed concurrently from different tasks.

I.e. what we care about in this case is interior mutability, where we can h=
ave
three cases:

  (1) There is no interior mutability, hence the type is Sync.

  (2) There is interior mutability, but there's also internal mechanisms ta=
king
      care of data races, hence the type is Sync.

  (3) There is interior mutability and nothing prevents data races, hence t=
he
      type is not Sync.

I feel like only case (2) really needs justification, because it needs to
explain how those "internal mechanisms" work.

Those abstractions do not have any interior mutability, hence I think what =
we
have is enough. Does that make sense?