把Device trait的set_class改为设置Weak指针，以避免循环引用问题。 (#666)

kernel/src/driver/video/fbdev/base/fbcon/mod.rs

@@ -197,7 +197,7 @@ impl Device for FbConsoleDevice {
         self.inner.lock().bus.clone()
     }
 
-    fn set_class(&self, _class: Option<Arc<dyn Class>>) {
+    fn set_class(&self, _class: Option<Weak<dyn Class>>) {
         // 不允许修改
         kwarn!("fbcon's class can not be changed");
     }

kernel/src/driver/video/fbdev/base/fbmem.rs

@@ -343,7 +343,7 @@ impl Device for FbDevice {
     fn class(&self) -> Option<Arc<dyn Class>> {
         sys_class_graphics_instance().map(|ins| ins.clone() as Arc<dyn Class>)
     }
-    fn set_class(&self, _class: Option<Arc<dyn Class>>) {
+    fn set_class(&self, _class: Option<Weak<dyn Class>>) {
         // do nothing
     }
 

kernel/src/driver/video/fbdev/vesafb.rs

@@ -122,7 +122,7 @@ impl VesaFb {
 #[derive(Debug)]
 struct InnerVesaFb {
     bus: Option<Weak<dyn Bus>>,
-    class: Option<Arc<dyn Class>>,
+    class: Option<Weak<dyn Class>>,
     driver: Option<Weak<dyn Driver>>,
     kern_inode: Option<Arc<KernFSInode>>,
     parent: Option<Weak<dyn KObject>>,
@@ -189,10 +189,22 @@ impl Device for VesaFb {
         self.inner.lock().bus = bus;
     }
 
-    fn set_class(&self, class: Option<Arc<dyn Class>>) {
+    fn set_class(&self, class: Option<Weak<dyn Class>>) {
         self.inner.lock().class = class;
     }
 
+    fn class(&self) -> Option<Arc<dyn Class>> {
+        let mut guard = self.inner.lock();
+
+        let r = guard.class.clone()?.upgrade();
+        if r.is_none() {
+            // 为了让弱引用失效
+            guard.class = None;
+        }
+
+        return r;
+    }
+
     fn driver(&self) -> Option<Arc<dyn Driver>> {
         self.inner.lock().driver.clone()?.upgrade()
     }