rcuref: Provide rcuref_is_dead()

rcuref_read() returns the number of references that are currently held.
If 0 is returned then it is not safe to assume that the object ca be
scheduled for deconstruction because it is marked DEAD. This happens if
the return value of rcuref_put() is ignored and assumptions are made.

If 0 is returned then the counter transitioned from 0 to RCUREF_NOREF.
If rcuref_put() did not return to the caller then the counter did not
yet transition from RCUREF_NOREF to RCUREF_DEAD. This means that there
is still a chance that the counter will transition from RCUREF_NOREF to
0 meaning it is still valid and must not be deconstructed. In this brief
window rcuref_read() will return 0.

Provide rcuref_is_dead() to determine if the counter is marked as
RCUREF_DEAD.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250416162921.513656-2-bigeasy@linutronix.de

diff --git a/include/linux/rcuref.h b/include/linux/rcuref.h
index 6322d8c1c6b4294abb794b4e0af739f1b0caedd9..2fb2af6d982497d346c821863aa366c9082526bc 100644 (file)
--- a/include/linux/rcuref.h
+++ b/include/linux/rcuref.h
@@ -30,7 +30,11 @@ static inline void rcuref_init(rcuref_t *ref, unsigned int cnt)
  * rcuref_read - Read the number of held reference counts of a rcuref
  * @ref:       Pointer to the reference count
  *
- * Return: The number of held references (0 ... N)
+ * Return: The number of held references (0 ... N). The value 0 does not
+ * indicate that it is safe to schedule the object, protected by this reference
+ * counter, for deconstruction.
+ * If you want to know if the reference counter has been marked DEAD (as
+ * signaled by rcuref_put()) please use rcuread_is_dead().
  */
 static inline unsigned int rcuref_read(rcuref_t *ref)
 {
@@ -40,6 +44,22 @@ static inline unsigned int rcuref_read(rcuref_t *ref)
        return c >= RCUREF_RELEASED ? 0 : c + 1;
 }
 
+/**
+ * rcuref_is_dead -    Check if the rcuref has been already marked dead
+ * @ref:               Pointer to the reference count
+ *
+ * Return: True if the object has been marked DEAD. This signals that a previous
+ * invocation of rcuref_put() returned true on this reference counter meaning
+ * the protected object can safely be scheduled for deconstruction.
+ * Otherwise, returns false.
+ */
+static inline bool rcuref_is_dead(rcuref_t *ref)
+{
+       unsigned int c = atomic_read(&ref->refcnt);
+
+       return (c >= RCUREF_RELEASED) && (c < RCUREF_NOREF);
+}
+
 extern __must_check bool rcuref_get_slowpath(rcuref_t *ref);
 
 /**