Message-ID: <3b557f61-cead-4568-b2b4-4a56c4cbff52@proton.me>
Date: Fri, 30 Aug 2024 13:21:55 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...sung.com>, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rust: add global lock support

On 30.08.24 07:34, Alice Ryhl wrote:
> On Thu, Aug 29, 2024 at 8:17 PM Benno Lossin <benno.lossin@...ton.me> wrote:
>>
>> On 27.08.24 10:41, Alice Ryhl wrote:
>>> We don't currently have any support for global locks in Rust, however
>>> they are very useful and I have needed to work around this limitation
>>> several times. My workarounds generally involve initializing the mutex
>>> in the module's init function, and this workaround is reflected here.
>>
>> I would not exactly call this a "workaround". If your use-case is
>> covered by putting a `Mutex`, then I would prefer it. Or did you need
>> additional ugly code to access it?
> 
> Not sure what you mean by "putting a Mutex" but the workaround is

Oh sorry, seems like I forgot to write the rest of that... Let me try
again: If your use-case is covered by putting a `Mutex` inside of the
type that implements `Module`, then I think you should do that instead
of using a global. (you need the inplace module patch for that)

> really gross and involves defining a whole struct to make types Sync
> and so on. Unlike binder, this patch has access to private fields of
> Lock, so it can do it in a more nice way. You can find it in the
> Binder RFC if you search for "context_global".
> https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-2-08ba9197f637@google.com/#Z31drivers:android:context.rs

Oh I remember this... Yeah I agree that is ugly, but it is not the
workaround that I imagined when you wrote "initializing the mutex in the
module's init function". There I was thinking of what I wrote above.

This might just be me misunderstanding that, but if you want to improve
it, then you could mention that the mutex is still a static.

>>> Due to the initialization requirement, constructing a global mutex is
>>> unsafe with the current approach. In the future, it would be really nice
>>> to support global mutexes that don't need to be initialized, which would
>>> make them safe. Unfortunately, this is not possible today because
>>> bindgen refuses to expose __ARCH_SPIN_LOCK_UNLOCKED to Rust as a
>>> compile-time constant. It just generates an `extern "C"` global
>>> reference instead.
>>
>> Ideally, we would have support for static initialization in pinned-init.
> 
> I don't think traits work with const today, so pin-init would need an
> entirely different mechanism? If you're talking about using

Oh yeah I forgot that that got scratched some time ago.

> CONSTRUCTORS, then I think it's an undesirable solution. C code can

No, I was thinking that the initializer is run at const eval and then
the result is put into the binary.

> define static mutexes without load-time initialization hooks. We
> should be able to do the same.

Agreed.

>>> On most architectures, we could initialize the lock to just contain all
>>> zeros. A possible improvement would be to create a Kconfig constant
>>> that is set whenever the current architecture uses all zeros for the
>>> initializer and have `unsafe_const_init` be a no-op on those
>>> architectures. We could also provide a safe const initializer that is
>>> only available when that Kconfig option is set.
>>
>> I am not sure if the two branches (depending on the config) will be a
>> good idea. We don't save on `unsafe` and only increase code complexity.
>> The no-op sounds like a better idea to me.
> 
> You mean put the logic here instead in the downstream user? I agree.

I meant that

    #[cfg(ZERO_LOCK_INIT)]
    static MY_MUTEX: Mutex<()> = Mutex::new_zeroed();

    #[cfg(not(ZERO_LOCK_INIT))]
    // SAFETY: ...
    static MY_MUTEX: Mutex<()> = unsafe { Mutex::unsafe_const_new() };


    module_init() {
        #[cfg(not(ZERO_LOCK_INIT))]
        {
            // SAFETY: ...
            unsafe { MY_MUTEX.unsafe_const_init() };
        }
    }

is significantly worse compared to just

    // SAFETY: ...
    static MY_MUTEX: Mutex<()> = unsafe { Mutex::unsafe_const_new() };


    module_init() {
        // SAFETY: ...
        unsafe { MY_MUTEX.unsafe_const_init() };
        //                ^^^^^^^^^^^^^^^^^
        //                if ZERO_LOCK_INIT, then this is a no-op.
    }

>>> For architectures that don't use all-zeros for the unlocked case, we
>>> will most likely have to hard-code the correct representation on the
>>> Rust side.
>>
>> You mean in `unsafe_const_init`?
> 
> No, I mean we would have `unsafe_const_new` directly set `state` to
> the right value and let `unsafe_const_init` be a no-op.

But how do you set the right value of a list_head? The value will be
moved.

>>> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
>>> ---
>>> Changes in v2:
>>> - Require `self: Pin<&Self>` and recommend `Pin::static_ref`.
>>> - Other doc improvements including new example.
>>> - Link to v1: https://lore.kernel.org/r/20240826-static-mutex-v1-1-a14ee71561f3@google.com
>>> ---
>>>  rust/kernel/sync/lock.rs | 64 +++++++++++++++++++++++++++++++++++++++++++++++-
>>>  1 file changed, 63 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
>>> index f6c34ca4d819..cfc5e160d78c 100644
>>> --- a/rust/kernel/sync/lock.rs
>>> +++ b/rust/kernel/sync/lock.rs
>>> @@ -7,7 +7,7 @@
>>>
>>>  use super::LockClassKey;
>>>  use crate::{init::PinInit, pin_init, str::CStr, types::Opaque, types::ScopeGuard};
>>> -use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned};
>>> +use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned, pin::Pin};
>>>  use macros::pin_data;
>>>
>>>  pub mod mutex;
>>> @@ -117,6 +117,68 @@ pub fn new(t: T, name: &'static CStr, key: &'static LockClassKey) -> impl PinIni
>>>              }),
>>>          })
>>>      }
>>> +
>>> +    /// Create a global lock that has not yet been initialized.
>>> +    ///
>>
>> Could you add a paragraph that explains that
> 
> Explains that what?

... this is not the usual way to create a `Lock`, use this only when
creating a global, `static` lock. For all other purposes, use
`new_<lock-type>`.

>>> +    /// Since global locks is not yet fully supported, this method implements global locks by
>>> +    /// requiring you to initialize them before you start using it. Usually this is best done in
>>> +    /// the module's init function.
>>> +    ///
>>> +    /// # Examples
>>> +    ///
>>
>> I would preface this example with "Instead of [`Mutex<T>`], you can use
>> any other lock.".
> 
> I don't love that wording. How about something along these lines?
> "This example uses a Mutex, but this works with any other lock
> including spin locks."

Sure.

>>> +    /// ```
>>> +    /// use kernel::sync::Mutex;
>>> +    ///
>>> +    /// // SAFETY: We initialize the mutex before first use.
>>> +    /// static MY_MUTEX: Mutex<()> = unsafe { Mutex::unsafe_const_new(()) };
>>> +    ///
>>> +    /// // For the sake of this example, assume that this is the module initializer.
>>
>> Why not actually provide a module initializer?
> 
> Can I put a `module!` macro inside a kunit test? I assumed that I couldn't.

I think if you wrap it in another `mod`, then it should work, but I
might be wrong.

>>> +    /// fn module_init() {
>>> +    ///     // SAFETY:
>>> +    ///     // * `MY_MUTEX` was created using `unsafe_const_new`.
>>> +    ///     // * This call is in the module initializer, which doesn't runs more than once.
>>> +    ///     unsafe {
>>> +    ///         core::pin::Pin::static_ref(&MY_MUTEX)
>>
>> I would put this into a let binding, that way the formatting will also
>> be nicer.
> 
> Ok.
> 
>>> +    ///             .unsafe_const_init(kernel::c_str!("MY_MUTEX"), kernel::static_lock_class!())
>>> +    ///     };
>>> +    /// }
>>> +    /// ```
>>> +    ///
>>> +    /// # Safety
>>> +    ///
>>> +    /// You must call [`unsafe_const_init`] before calling any other method on this lock.
>>> +    ///
>>> +    /// [`unsafe_const_init`]: Self::unsafe_const_init
>>> +    pub const unsafe fn unsafe_const_new(t: T) -> Self {
>>
>> I am not sure on this name, I don't think we have any functions with
>> `unsafe` in it (and `std` also doesn't). How about `new_uninitialized`?
>>
>> Although that might be confusing, since it does actually take a value...
> 
> Hmm. Any other ideas? const_new_need_manual_init?

Hmm that seems too long... `new_static_uninit`? I don't think that
`const` belongs in the name either, since you wouldn't use it in a
`const` (but sure it is used in const contexts, but I find putting it in
the name confusing).

---
Cheers,
Benno

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