Re: [PATCH v4] rust: add global lock support

[Andreas Hindborg <a.hindborg@kernel.org>]

Alice Ryhl <aliceryhl@google.com> writes:

> On Thu, Oct 10, 2024 at 3:57 PM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>>
>> Hi Alice,
>>
>> Alice Ryhl <aliceryhl@google.com> writes:
>>
>> > Add support for creating global variables that are wrapped in a mutex or
>> > spinlock. Optionally, the macro can generate a special LockedBy type
>> > that does not require a runtime check.
>> >
>> > The implementation here is intended to replace the global mutex
>> > workaround found in the Rust Binder RFC [1]. In both cases, the global
>> > lock must be initialized before first use. The macro is unsafe to use
>> > for the same reason.
>> >
>> > The separate initialization step is required because it is tricky to
>> > access the value of __ARCH_SPIN_LOCK_UNLOCKED from Rust. Doing so will
>> > require changes to the C side. That change will happen as a follow-up to
>> > this patch.
>>
>> Why is this a challenge? It seems to work with locks that are not
>> global.
>
> Because normal locks are not initialized in a const expression.
>
>> > diff --git a/rust/kernel/sync/lock/global.rs b/rust/kernel/sync/lock/global.rs
>> > new file mode 100644
>> > index 000000000000..fc02fac864f6
>> > --- /dev/null
>> > +++ b/rust/kernel/sync/lock/global.rs
>> > @@ -0,0 +1,260 @@
>> > +// SPDX-License-Identifier: GPL-2.0
>> > +
>> > +// Copyright (C) 2024 Google LLC.
>> > +
>> > +//! Support for defining statics containing locks.
>> > +
>> > +/// Defines a global lock.
>> > +///
>> > +/// Supports the following options:
>> > +///
>> > +/// * `value` specifies the initial value in the global lock.
>> > +/// * `wrapper` specifies the name of the wrapper struct.
>>
>> Could you add an example to demonstrate when using `wrapper` option
>> would be useful?
>
> Probably only guard and locked_by are useful, but I think you need to
> give the wrapper a name to reasonably use guard/locked_by.

Could you expand the example to show this, or would it become too
long?

BR Andreas