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

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Message-ID: <aAkfOe5ZDUgIawyU@yury>
Date: Wed, 23 Apr 2025 13:11:21 -0400
From: Yury Norov <yury.norov@...il.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Alice Ryhl <aliceryhl@...gle.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 09:30:51AM -0700, Boqun Feng wrote:
> 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.

Not sure what you mean. Atomic set_bit() and clear() bit are often
implemented in asm, and there's no acquire-release semantic.