Re: [PATCH 1/3] rust: sync: Add Lock::is_locked()

Boqun Feng
2024-11-20T23:53:57Z

On Wed, Nov 20, 2024 at 05:30:41PM -0500, Lyude Paul wrote:
> Now that we've added a Lock::from_raw() function and exposed Guard::new(),
> it would be good to actually add the ability to assert the current state
> of a lock to ensure correctness for unsafe code using these functions.
>
> To do so, let's add Lock::is_locked() which simply returns whether or not a
> Lock is acquired. We'll use this in the next few commits to add some debug
> assertions.
>
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> ---
> rust/helpers/spinlock.c | 5 +++++
> rust/kernel/sync/lock.rs | 18 ++++++++++++++++++
> rust/kernel/sync/lock/mutex.rs | 5 +++++
> rust/kernel/sync/lock/spinlock.rs | 5 +++++
> 4 files changed, 33 insertions(+)
>
> diff --git a/rust/helpers/spinlock.c b/rust/helpers/spinlock.c
> index b7b0945e8b3cb..90216a69e3ea1 100644
> --- a/rust/helpers/spinlock.c
> +++ b/rust/helpers/spinlock.c
> @@ -26,3 +26,8 @@ int rust_helper_spin_trylock(spinlock_t *lock)
> {
> return spin_trylock(lock);
> }
> +
> +bool rust_helper_spin_is_locked(spinlock_t *lock)
> +{
> + return spin_is_locked(lock);
> +}
> diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> index 139f17f2ec86b..542f846ac02b2 100644
> --- a/rust/kernel/sync/lock.rs
> +++ b/rust/kernel/sync/lock.rs
> @@ -85,6 +85,13 @@ unsafe fn relock(ptr: *mut Self::State, guard_state: &mut Self::GuardState) {
> // SAFETY: The safety requirements ensure that the lock is initialised.
> *guard_state = unsafe { Self::lock(ptr) };
> }
> +
> + /// Returns whether or not the lock is currently acquired.
> + ///
> + /// # Safety
> + ///
> + /// Callers must ensure that `ptr` is a valid initialised pointer to this lock type.
> + unsafe fn is_locked(ptr: *mut Self::State) -> bool;
> }
>
> /// A mutual exclusion primitive.
> @@ -170,6 +177,17 @@ pub fn try_lock(&self) -> Option<Guard<'_, T, B>> {
> // that `init` was called.
> unsafe { B::try_lock(self.state.get()).map(|state| Guard::new(self, state)) }
> }
> +
> + /// Return whether or not the lock is currently acquired.
> + ///
> + /// Keep in mind that this function is inherently racy: a lock could immediately be acquired or
> + /// released after this function returns. As such, the return value from this function should be
> + /// treated as a snapshot for debugging purposes.

Then why don't we use the function provided by lockdep? I.e.
lockdep_is_held() and its friends?

Regards,
Boqun

> + pub fn is_locked(&self) -> bool {

> + // SAFETY: The constructor of the type calls `init`, so the existence of the object proves
> + // that `init` was called.
> + unsafe { B::is_locked(self.state.get()) }
> + }
> }
>
> /// A lock guard.
> diff --git a/rust/kernel/sync/lock/mutex.rs b/rust/kernel/sync/lock/mutex.rs
> index 0e946ebefce12..f21b1f14cbe1b 100644
> --- a/rust/kernel/sync/lock/mutex.rs
> +++ b/rust/kernel/sync/lock/mutex.rs
> @@ -126,4 +126,9 @@ unsafe fn try_lock(ptr: *mut Self::State) -> Option<Self::GuardState> {
> None
> }
> }
> +
> + unsafe fn is_locked(ptr: *mut Self::State) -> bool {
> + // SAFETY: The `ptr` pointer is guaranteed to be valid and initialized before use.
> + unsafe { bindings::mutex_is_locked(ptr) }
> + }
> }
> diff --git a/rust/kernel/sync/lock/spinlock.rs b/rust/kernel/sync/lock/spinlock.rs
> index 9f4d128bed983..cfccf5e900b80 100644
> --- a/rust/kernel/sync/lock/spinlock.rs
> +++ b/rust/kernel/sync/lock/spinlock.rs
> @@ -125,4 +125,9 @@ unsafe fn try_lock(ptr: *mut Self::State) -> Option<Self::GuardState> {
> None
> }
> }
> +
> + unsafe fn is_locked(ptr: *mut Self::State) -> bool {
> + // SAFETY: The `ptr` pointer is guaranteed to be valid and initialized before use.
> + unsafe { bindings::spin_is_locked(ptr) }
> + }
> }
> --
> 2.47.0
>