linux-kernel - Re: [PATCH] rust: sync: lock: Add an example for Guard::lock

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Message-ID: <304505cb-9f68-4d34-b4f1-7d703baba012@proton.me>
Date: Mon, 24 Feb 2025 10:06:35 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Andreas Hindborg <a.hindborg@...nel.org>, Boqun Feng <boqun.feng@...il.com>
Cc: linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>
Subject: Re: [PATCH] rust: sync: lock: Add an example for Guard::lock_ref()

On 24.02.25 09:08, Andreas Hindborg wrote:
> Boqun Feng <boqun.feng@...il.com> writes:
> 
>> To provide examples on usage of `Guard::lock_ref()` along with the unit
>> test, an "assert a lock is held by a guard" example is added.
>>
>> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
>> ---
>> This depends on Alice's patch:
>>
>> 	https://lore.kernel.org/all/20250130-guard-get-lock-v1-1-8ed87899920a@google.com/
>>
>> I'm also OK to fold this in if Alice thinks it's fine.
>>
>>  rust/kernel/sync/lock.rs | 24 ++++++++++++++++++++++++
>>  1 file changed, 24 insertions(+)
>>
>> diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
>> index 3701fac6ebf6..6d868e35b0a3 100644
>> --- a/rust/kernel/sync/lock.rs
>> +++ b/rust/kernel/sync/lock.rs
>> @@ -201,6 +201,30 @@ unsafe impl<T: Sync + ?Sized, B: Backend> Sync for Guard<'_, T, B> {}
>>
>>  impl<'a, T: ?Sized, B: Backend> Guard<'a, T, B> {
>>      /// Returns the lock that this guard originates from.
>> +    ///
>> +    /// # Examples
>> +    ///
>> +    /// The following example shows how to use [`Guard::lock_ref()`] to assert the corresponding
>> +    /// lock is held.
>> +    ///
>> +    /// ```
>> +    /// # use kernel::{new_spinlock, stack_pin_init, sync::lock::{Backend, Guard, Lock}};
>> +    ///
>> +    /// fn assert_held<T, B: Backend>(guard: &Guard<'_, T, B>, lock: &Lock<T, B>) {
>> +    ///     // Address-equal means the same lock.
>> +    ///     assert!(core::ptr::eq(guard.lock_ref(), lock));
>> +    /// }
> 
> This seems super useful. Perhaps add this method as part of the lock api
> instead of just having it in the example?

I don't think it should be an assert. Instead make it return a
`Result<(), ()>`. (or create better named unit error types)

---
Cheers,
Benno