linux-kernel - Re: [PATCH v6 3/7] rust: implement `WwMutex`, `WwAcquireCtx` and `WwMutexGuard`

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Message-Id: <E98866AA-AE29-432E-B6D3-4625103B8D07@collabora.com>
Date: Fri, 5 Sep 2025 16:03:02 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Onur Özkan <work@...rozkan.dev>
Cc: rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org,
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Subject: Re: [PATCH v6 3/7] rust: implement `WwMutex`, `WwAcquireCtx` and
 `WwMutexGuard`

[…]

> 
>> +///
>> +/// # Examples
>> +///
>> +/// ## Basic Usage
>> +///
>> +/// ```
>> +/// use kernel::c_str;
>> +/// use kernel::sync::Arc;
>> +/// use kernel::sync::lock::ww_mutex::{WwClass, WwAcquireCtx, WwMutex };
>> +/// use pin_init::stack_pin_init;
>> +///
>> +/// stack_pin_init!(let class = WwClass::new_wound_wait(c_str!("buffer_class")));
>> +/// let mutex = Arc::pin_init(WwMutex::new(42, &class), GFP_KERNEL)?;
>> +///
>> +/// let ctx = KBox::pin_init(WwAcquireCtx::new(&class), GFP_KERNEL)?;
>> +///
>> +/// let guard = ctx.lock(&mutex)?;
>> +/// assert_eq!(*guard, 42);
>> +///
>> +/// # Ok::<(), Error>(())
>> +/// ```
>> +///
>> +/// ## Multiple Locks
>> +///
>> +/// ```
>> +/// use kernel::c_str;
>> +/// use kernel::prelude::*;
>> +/// use kernel::sync::Arc;
>> +/// use kernel::sync::lock::ww_mutex::{WwClass, WwAcquireCtx, WwMutex};
>> +/// use pin_init::stack_pin_init;
>> +///
>> +/// stack_pin_init!(let class = WwClass::new_wait_die(c_str!("resource_class")));
>> +/// let mutex_a = Arc::pin_init(WwMutex::new("Resource A", &class), GFP_KERNEL)?;
>> +/// let mutex_b = Arc::pin_init(WwMutex::new("Resource B", &class), GFP_KERNEL)?;
>> +///
>> +/// let ctx = KBox::pin_init(WwAcquireCtx::new(&class), GFP_KERNEL)?;
>> +///
>> +/// // Try to acquire both locks.
>> +/// let guard_a = match ctx.lock(&mutex_a) {
>> +///     Ok(guard) => guard,
>> +///     Err(e) if e == EDEADLK => {
>> +///         // Deadlock detected, use slow path.
> 
> You must release all other locks before calling this, except there aren’t any taken in your example.
> 
> You should perhaps add a release_all() function to the context.
> 

By the way, if we need a context in the first place to lock, it makes sense to
release_all() once this context is dropped.

— Daniel