[PATCH v2 1/2] devres: add devm_remove_action_nowarn()

[PATCH v2 1/2] devres: add devm_remove_action_nowarn()

[Danilo Krummrich]

devm_remove_action() warns if the action to remove does not exist
(anymore).

The Rust devres abstraction, however, has a use-case to call
devm_remove_action() at a point where it can't be guaranteed that the
corresponding action hasn't been released yet.

In particular, an instance of `Devres<T>` may be dropped after the
action has been released. So far, `Devres<T>` worked around this by
keeping the inner type alive.

Hence, add devm_remove_action_nowarn(), which returns an error code if
the action has been removed already.

A subsequent patch uses devm_remove_action_nowarn() to remove the action
when `Devres<T>` is dropped.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
v2: clarify area of use of devm_remove_action_nowarn()
---
 drivers/base/devres.c  | 23 ++++++++++++++++++-----
 include/linux/device.h | 18 +++++++++++++++++-
 2 files changed, 35 insertions(+), 6 deletions(-)

diff --git a/drivers/base/devres.c b/drivers/base/devres.c
index 2152eec0c135..93e7779ef21e 100644
--- a/drivers/base/devres.c
+++ b/drivers/base/devres.c
@@ -750,25 +750,38 @@ int __devm_add_action(struct device *dev, void (*action)(void *), void *data, co
 EXPORT_SYMBOL_GPL(__devm_add_action);
 
 /**
- * devm_remove_action() - removes previously added custom action
+ * devm_remove_action_nowarn() - removes previously added custom action
  * @dev: Device that owns the action
  * @action: Function implementing the action
  * @data: Pointer to data passed to @action implementation
  *
  * Removes instance of @action previously added by devm_add_action().
  * Both action and data should match one of the existing entries.
+ *
+ * In contrast to devm_remove_action(), this function does not WARN() if no
+ * entry could have been found.
+ *
+ * This should only be used if the action is contained in an object with
+ * independent lifetime management, e.g. the Devres rust abstraction.
+ *
+ * Causing the warning from regular driver code most likely indicates an abuse
+ * of the devres API.
+ *
+ * Returns: 0 on success, -ENOENT if no entry could have been found.
  */
-void devm_remove_action(struct device *dev, void (*action)(void *), void *data)
+int devm_remove_action_nowarn(struct device *dev,
+			      void (*action)(void *),
+			      void *data)
 {
 	struct action_devres devres = {
 		.data = data,
 		.action = action,
 	};
 
-	WARN_ON(devres_destroy(dev, devm_action_release, devm_action_match,
-			       &devres));
+	return devres_destroy(dev, devm_action_release, devm_action_match,
+			      &devres);
 }
-EXPORT_SYMBOL_GPL(devm_remove_action);
+EXPORT_SYMBOL_GPL(devm_remove_action_nowarn);
 
 /**
  * devm_release_action() - release previously added custom action
diff --git a/include/linux/device.h b/include/linux/device.h
index 667cb6db9019..6879d5e8ac3d 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -399,7 +399,23 @@ void __iomem *devm_of_iomap(struct device *dev,
 #endif
 
 /* allows to add/remove a custom action to devres stack */
-void devm_remove_action(struct device *dev, void (*action)(void *), void *data);
+int devm_remove_action_nowarn(struct device *dev, void (*action)(void *), void *data);
+
+/**
+ * devm_remove_action() - removes previously added custom action
+ * @dev: Device that owns the action
+ * @action: Function implementing the action
+ * @data: Pointer to data passed to @action implementation
+ *
+ * Removes instance of @action previously added by devm_add_action().
+ * Both action and data should match one of the existing entries.
+ */
+static inline
+void devm_remove_action(struct device *dev, void (*action)(void *), void *data)
+{
+	WARN_ON(devm_remove_action_nowarn(dev, action, data));
+}
+
 void devm_release_action(struct device *dev, void (*action)(void *), void *data);
 
 int __devm_add_action(struct device *dev, void (*action)(void *), void *data, const char *name);

base-commit: 06e843bbbf2107463249ea6f6b1a736f5647e24a
-- 
2.47.1

[PATCH v2 2/2] rust: devres: remove action in `Devres::drop`

[Danilo Krummrich]

So far `DevresInner` is kept alive, even if `Devres` is dropped until
the devres callback is executed to avoid a WARN() when the action has
been released already.

With the introduction of devm_remove_action_nowarn() we can remove the
action in `Devres::drop`, handle the case where the action has been
released already and hence also free `DevresInner`.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
v2:
  - remove unnecessary call to revoke
  - change argument of remove_action() from `&Self` to `&Arc<Self>`
---
 rust/kernel/devres.rs | 47 ++++++++++++++++++++++++++++++++-----------
 1 file changed, 35 insertions(+), 12 deletions(-)

diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
index 9c9dd39584eb..942376f6f3af 100644
--- a/rust/kernel/devres.rs
+++ b/rust/kernel/devres.rs
@@ -10,15 +10,19 @@
     bindings,
     device::Device,
     error::{Error, Result},
+    ffi::c_void,
     prelude::*,
     revocable::Revocable,
     sync::Arc,
+    types::ARef,
 };
 
 use core::ops::Deref;
 
 #[pin_data]
 struct DevresInner<T> {
+    dev: ARef<Device>,
+    callback: unsafe extern "C" fn(*mut c_void),
     #[pin]
     data: Revocable<T>,
 }
@@ -98,6 +102,8 @@ impl<T> DevresInner<T> {
     fn new(dev: &Device, data: T, flags: Flags) -> Result<Arc<DevresInner<T>>> {
         let inner = Arc::pin_init(
             pin_init!( DevresInner {
+                dev: dev.into(),
+                callback: Self::devres_callback,
                 data <- Revocable::new(data),
             }),
             flags,
@@ -109,9 +115,8 @@ fn new(dev: &Device, data: T, flags: Flags) -> Result<Arc<DevresInner<T>>> {
 
         // SAFETY: `devm_add_action` guarantees to call `Self::devres_callback` once `dev` is
         // detached.
-        let ret = unsafe {
-            bindings::devm_add_action(dev.as_raw(), Some(Self::devres_callback), data as _)
-        };
+        let ret =
+            unsafe { bindings::devm_add_action(dev.as_raw(), Some(inner.callback), data as _) };
 
         if ret != 0 {
             // SAFETY: We just created another reference to `inner` in order to pass it to
@@ -124,6 +129,32 @@ fn new(dev: &Device, data: T, flags: Flags) -> Result<Arc<DevresInner<T>>> {
         Ok(inner)
     }
 
+    fn as_ptr(&self) -> *const Self {
+        self as _
+    }
+
+    fn remove_action(this: &Arc<Self>) {
+        // SAFETY:
+        // - `self.inner.dev` is a valid `Device`,
+        // - the `action` and `data` pointers are the exact same ones as given to devm_add_action()
+        //   previously,
+        // - `self` is always valid, even if the action has been released already.
+        let ret = unsafe {
+            bindings::devm_remove_action_nowarn(
+                this.dev.as_raw(),
+                Some(this.callback),
+                this.as_ptr() as _,
+            )
+        };
+
+        if ret == 0 {
+            // SAFETY: We leaked an `Arc` reference to devm_add_action() in `DevresInner::new`; if
+            // devm_remove_action_nowarn() was successful we can (and have to) claim back ownership
+            // of this reference.
+            let _ = unsafe { Arc::from_raw(this.as_ptr()) };
+        }
+    }
+
     #[allow(clippy::missing_safety_doc)]
     unsafe extern "C" fn devres_callback(ptr: *mut kernel::ffi::c_void) {
         let ptr = ptr as *mut DevresInner<T>;
@@ -165,14 +196,6 @@ fn deref(&self) -> &Self::Target {
 
 impl<T> Drop for Devres<T> {
     fn drop(&mut self) {
-        // Revoke the data, such that it gets dropped already and the actual resource is freed.
-        //
-        // `DevresInner` has to stay alive until the devres callback has been called. This is
-        // necessary since we don't know when `Devres` is dropped and calling
-        // `devm_remove_action()` instead could race with `devres_release_all()`.
-        //
-        // SAFETY: When `drop` runs, it's guaranteed that nobody is accessing the revocable data
-        // anymore, hence it is safe not to wait for the grace period to finish.
-        unsafe { self.revoke_nosync() };
+        DevresInner::remove_action(&self.0);
     }
 }
-- 
2.47.1