Merge tag 'locking-urgent-2025-02-28' of git://git.kernel.org/pub/scm…

…/linux/kernel/git/tip/tip

Pull locking fix from Ingo Molnar:
 "Fix an rcuref_put() slowpath race"

* tag 'locking-urgent-2025-02-28' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  rcuref: Plug slowpath race in rcuref_put()

include/linux/rcuref.h

@@ -71,27 +71,30 @@ static inline __must_check bool rcuref_get(rcuref_t *ref)
 	return rcuref_get_slowpath(ref);
 }
 
-extern __must_check bool rcuref_put_slowpath(rcuref_t *ref);
+extern __must_check bool rcuref_put_slowpath(rcuref_t *ref, unsigned int cnt);
 
 /*
  * Internal helper. Do not invoke directly.
  */
 static __always_inline __must_check bool __rcuref_put(rcuref_t *ref)
 {
+	int cnt;
+
 	RCU_LOCKDEP_WARN(!rcu_read_lock_held() && preemptible(),
 			 "suspicious rcuref_put_rcusafe() usage");
 	/*
 	 * Unconditionally decrease the reference count. The saturation and
 	 * dead zones provide enough tolerance for this.
 	 */
-	if (likely(!atomic_add_negative_release(-1, &ref->refcnt)))
+	cnt = atomic_sub_return_release(1, &ref->refcnt);
+	if (likely(cnt >= 0))
 		return false;
 
 	/*
 	 * Handle the last reference drop and cases inside the saturation
 	 * and dead zones.
 	 */
-	return rcuref_put_slowpath(ref);
+	return rcuref_put_slowpath(ref, cnt);
 }
 
 /**

lib/rcuref.c

@@ -220,6 +220,7 @@ EXPORT_SYMBOL_GPL(rcuref_get_slowpath);
 /**
  * rcuref_put_slowpath - Slowpath of __rcuref_put()
  * @ref:	Pointer to the reference count
+ * @cnt:	The resulting value of the fastpath decrement
  *
  * Invoked when the reference count is outside of the valid zone.
  *
@@ -233,10 +234,8 @@ EXPORT_SYMBOL_GPL(rcuref_get_slowpath);
  *	with a concurrent get()/put() pair. Caller is not allowed to
  *	deconstruct the protected object.
  */
-bool rcuref_put_slowpath(rcuref_t *ref)
+bool rcuref_put_slowpath(rcuref_t *ref, unsigned int cnt)
 {
-	unsigned int cnt = atomic_read(&ref->refcnt);
-
 	/* Did this drop the last reference? */
 	if (likely(cnt == RCUREF_NOREF)) {
 		/*