linux-kernel - Re: [PATCH v7 0/5] rust: adds Bitmap API, ID pool and bindings

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Message-ID: <aAkVu8Uf3J8F25fY@Mac.home>
Date: Wed, 23 Apr 2025 09:30:51 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Yury Norov <yury.norov@...il.com>, Burak Emir <bqe@...gle.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Trevor Gross <tmgross@...ch.edu>, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 0/5] rust: adds Bitmap API, ID pool and bindings

On Wed, Apr 23, 2025 at 06:19:18PM +0200, Alice Ryhl wrote:
> On Wed, Apr 23, 2025 at 5:43 PM Yury Norov <yury.norov@...il.com> wrote:
> >
> > I received it twice - with timestamps 1:36 and 1:43. Assuming they are
> > identical, and ignoring the former.
> >
> > On Wed, Apr 23, 2025 at 01:43:32PM +0000, Burak Emir wrote:
> > > This series adds a Rust bitmap API for porting the approach from
> > > commit 15d9da3f818c ("binder: use bitmap for faster descriptor lookup")
> > > to Rust. The functionality in dbitmap.h makes use of bitmap and bitops.
> > >
> > > The Rust bitmap API provides a safe abstraction to underlying bitmap
> > > and bitops operations. For now, only includes method necessary for
> > > dbitmap.h, more can be added later. We perform bounds checks for
> > > hardening, violations are programmer errors that result in panics.
> > >
> > > We include set_bit_atomic and clear_bit_atomic operations. One has
> > > to avoid races with non-atomic operations, which is ensure by the
> > > Rust type system: either callers have shared references &bitmap in
> > > which case the mutations are atomic operations. Or there is a
> > > exclusive reference &mut bitmap, in which case there is no concurrent
> > > access.
> >
> > It's not about shared references only. One can take a mutable
> > reference, and still may have a race:
> >
> > CPU1                            CPU2
> >
> > take mut ref
> > bitmap.set() // non-atomic
> > put mut ref
> >                                 take mut ref
> >                                 bitmap.test() // read as 0
> > data propagated to memory
> >                                 bitmap.test() // read as 1
> >
> > To make this scenario impossible, either put or take mut ref
> > should imply global cache flush, because bitmap array is not
> > an internal data for the Bitmap class (only the pointer is).
> >
> > I already asked you to point me to the specification that states that
> > taking mutable reference implies flushing all the caches to the point
> > of coherency, but you didn't share it. And I doubt that compiler does
> > it, for the performance considerations.
> 
> The flushing of caches and so on *is* implied. It doesn't happen every
> time you take a mutable reference, but for you to be able to take a
> mut ref on CPU2 after releasing it on CPU1, there must be a flush
> somewhere in between.
> 

Yeah, and it's not just "flushing of caches", it's making CPU1's memory
operations on the object pointed by "mut ref" observable to CPU2. If
CPU1 and CPU2 sync with the a lock, then lock guarantees that, and if
CPU1 and CPU2 sync with a store-release+load-acquire, the
RELEASE-ACQUIRE ordering guarantees that as well.

Regards,
Boqun

> I can try to find docs for it ...
> 
> Alice