scsi: sysfs: Introduce sysfs_{un,}break_active_protection()

Introduce these two functions and export them such that the next patch
can add calls to these functions from the SCSI core.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

fs/sysfs/file.c

@@ -405,6 +405,50 @@ int sysfs_chmod_file(struct kobject *kobj, const struct attribute *attr,
 }
 EXPORT_SYMBOL_GPL(sysfs_chmod_file);
 
+/**
+ * sysfs_break_active_protection - break "active" protection
+ * @kobj: The kernel object @attr is associated with.
+ * @attr: The attribute to break the "active" protection for.
+ *
+ * With sysfs, just like kernfs, deletion of an attribute is postponed until
+ * all active .show() and .store() callbacks have finished unless this function
+ * is called. Hence this function is useful in methods that implement self
+ * deletion.
+ */
+struct kernfs_node *sysfs_break_active_protection(struct kobject *kobj,
+						  const struct attribute *attr)
+{
+	struct kernfs_node *kn;
+
+	kobject_get(kobj);
+	kn = kernfs_find_and_get(kobj->sd, attr->name);
+	if (kn)
+		kernfs_break_active_protection(kn);
+	return kn;
+}
+EXPORT_SYMBOL_GPL(sysfs_break_active_protection);
+
+/**
+ * sysfs_unbreak_active_protection - restore "active" protection
+ * @kn: Pointer returned by sysfs_break_active_protection().
+ *
+ * Undo the effects of sysfs_break_active_protection(). Since this function
+ * calls kernfs_put() on the kernfs node that corresponds to the 'attr'
+ * argument passed to sysfs_break_active_protection() that attribute may have
+ * been removed between the sysfs_break_active_protection() and
+ * sysfs_unbreak_active_protection() calls, it is not safe to access @kn after
+ * this function has returned.
+ */
+void sysfs_unbreak_active_protection(struct kernfs_node *kn)
+{
+	struct kobject *kobj = kn->parent->priv;
+
+	kernfs_unbreak_active_protection(kn);
+	kernfs_put(kn);
+	kobject_put(kobj);
+}
+EXPORT_SYMBOL_GPL(sysfs_unbreak_active_protection);
+
 /**
  * sysfs_remove_file_ns - remove an object attribute with a custom ns tag
  * @kobj: object we're acting for

include/linux/sysfs.h

@@ -237,6 +237,9 @@ int __must_check sysfs_create_files(struct kobject *kobj,
 				   const struct attribute **attr);
 int __must_check sysfs_chmod_file(struct kobject *kobj,
 				  const struct attribute *attr, umode_t mode);
+struct kernfs_node *sysfs_break_active_protection(struct kobject *kobj,
+						  const struct attribute *attr);
+void sysfs_unbreak_active_protection(struct kernfs_node *kn);
 void sysfs_remove_file_ns(struct kobject *kobj, const struct attribute *attr,
 			  const void *ns);
 bool sysfs_remove_file_self(struct kobject *kobj, const struct attribute *attr);
@@ -350,6 +353,17 @@ static inline int sysfs_chmod_file(struct kobject *kobj,
 	return 0;
 }
 
+static inline struct kernfs_node *
+sysfs_break_active_protection(struct kobject *kobj,
+			      const struct attribute *attr)
+{
+	return NULL;
+}
+
+static inline void sysfs_unbreak_active_protection(struct kernfs_node *kn)
+{
+}
+
 static inline void sysfs_remove_file_ns(struct kobject *kobj,
 					const struct attribute *attr,
 					const void *ns)