linux-kernel - Re: [PATCH v3 07/13] rust: lock: implement `IrqSaveBackend` for `SpinLock`

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Date:   Sun, 9 Apr 2023 13:48:26 -0300
From:   Martin Rodriguez Reboredo <yakoyoku@...il.com>
To:     Wedson Almeida Filho <wedsonaf@...il.com>,
        rust-for-linux@...r.kernel.org
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        linux-kernel@...r.kernel.org,
        Wedson Almeida Filho <walmeida@...rosoft.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v3 07/13] rust: lock: implement `IrqSaveBackend` for
 `SpinLock`

On 4/8/23 04:53, Wedson Almeida Filho wrote:
> From: Wedson Almeida Filho <walmeida@...rosoft.com>
> 
> This allows Rust code to use the `lock_irqsave` variant of spinlocks.
> 
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Waiman Long <longman@...hat.com>
> Signed-off-by: Wedson Almeida Filho <walmeida@...rosoft.com>
> ---
> v1 -> v2: No changes
> v2 -> v3: No changes
> 
>  rust/helpers.c                    | 16 +++++++++++++
>  rust/kernel/sync/lock/spinlock.rs | 38 ++++++++++++++++++++++++++-----
>  2 files changed, 48 insertions(+), 6 deletions(-)
> 
> diff --git a/rust/helpers.c b/rust/helpers.c
> index 05694e3f8f70..e42f5b446f92 100644
> --- a/rust/helpers.c
> +++ b/rust/helpers.c
> @@ -59,6 +59,22 @@ void rust_helper_spin_unlock(spinlock_t *lock)
>  }
>  EXPORT_SYMBOL_GPL(rust_helper_spin_unlock);
>  
> +unsigned long rust_helper_spin_lock_irqsave(spinlock_t *lock)
> +{
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(lock, flags);
> +
> +	return flags;
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_spin_lock_irqsave);
> +
> +void rust_helper_spin_unlock_irqrestore(spinlock_t *lock, unsigned long flags)
> +{
> +	spin_unlock_irqrestore(lock, flags);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_spin_unlock_irqrestore);
> +
>  refcount_t rust_helper_REFCOUNT_INIT(int n)
>  {
>  	return (refcount_t)REFCOUNT_INIT(n);
> diff --git a/rust/kernel/sync/lock/spinlock.rs b/rust/kernel/sync/lock/spinlock.rs
> index a52d20fc9755..34dec09a97c0 100644
> --- a/rust/kernel/sync/lock/spinlock.rs
> +++ b/rust/kernel/sync/lock/spinlock.rs
> @@ -61,6 +61,8 @@ macro_rules! new_spinlock {
>  /// assert_eq!(e.c, 10);
>  /// assert_eq!(e.d.lock().a, 20);
>  /// assert_eq!(e.d.lock().b, 30);
> +/// assert_eq!(e.d.lock_irqsave().a, 20);
> +/// assert_eq!(e.d.lock_irqsave().b, 30);
>  /// ```
>  ///
>  /// The following example shows how to use interior mutability to modify the contents of a struct
> @@ -79,6 +81,12 @@ macro_rules! new_spinlock {
>  ///     guard.a += 10;
>  ///     guard.b += 20;
>  /// }
> +///
> +/// fn example2(m: &SpinLock<Example>) {
> +///     let mut guard = m.lock_irqsave();
> +///     guard.a += 10;
> +///     guard.b += 20;
> +/// }
>  /// ```
>  ///
>  /// [`spinlock_t`]: ../../../../include/linux/spinlock.h
> @@ -90,7 +98,7 @@ pub struct SpinLockBackend;
>  // SAFETY: The underlying kernel `spinlock_t` object ensures mutual exclusion.
>  unsafe impl super::Backend for SpinLockBackend {
>      type State = bindings::spinlock_t;
> -    type GuardState = ();
> +    type GuardState = Option<core::ffi::c_ulong>;
>  
>      unsafe fn init(
>          ptr: *mut Self::State,
> @@ -105,12 +113,30 @@ unsafe impl super::Backend for SpinLockBackend {
>      unsafe fn lock(ptr: *mut Self::State) -> Self::GuardState {
>          // SAFETY: The safety requirements of this function ensure that `ptr` points to valid
>          // memory, and that it has been initialised before.
> -        unsafe { bindings::spin_lock(ptr) }
> +        unsafe { bindings::spin_lock(ptr) };
> +        None
>      }
>  
> -    unsafe fn unlock(ptr: *mut Self::State, _guard_state: &Self::GuardState) {
> -        // SAFETY: The safety requirements of this function ensure that `ptr` is valid and that the
> -        // caller is the owner of the mutex.
> -        unsafe { bindings::spin_unlock(ptr) }
> +    unsafe fn unlock(ptr: *mut Self::State, guard_state: &Self::GuardState) {
> +        match guard_state {
> +            // SAFETY: The safety requirements of this function ensure that `ptr` is valid and that
> +            // the caller is the owner of the mutex.
> +            Some(flags) => unsafe { bindings::spin_unlock_irqrestore(ptr, *flags) },
> +            // SAFETY: The safety requirements of this function ensure that `ptr` is valid and that
> +            // the caller is the owner of the mutex.
> +            None => unsafe { bindings::spin_unlock(ptr) },
> +        }
> +    }
> +}
> +
> +// SAFETY: The underlying kernel `spinlock_t` object ensures mutual exclusion. We use the `irqsave`
> +// variant of the C lock acquisition functions to disable interrupts and retrieve the original
> +// interrupt state, and the `irqrestore` variant of the lock release functions to restore the state
> +// in `unlock` -- we use the guard context to determine which method was used to acquire the lock.
> +unsafe impl super::IrqSaveBackend for SpinLockBackend {
> +    unsafe fn lock_irqsave(ptr: *mut Self::State) -> Self::GuardState {
> +        // SAFETY: The safety requirements of this function ensure that `ptr` points to valid
> +        // memory, and that it has been initialised before.
> +        Some(unsafe { bindings::spin_lock_irqsave(ptr) })
>      }
>  }

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@...il.com>