Add yield to avoid starvation

kernel/src/process/thread.rs

@@ -511,7 +511,10 @@ pub fn spawn(thread: Arc<Thread>) {
 
             thread.end_running(cx);
             if exit {
+                info!("thread {} stopped", thread.tid);
                 break;
+            } else {
+                yield_now().await;
             }
         }
     };
@@ -556,3 +559,28 @@ impl Future for PageTableSwitchWrapper {
         res
     }
 }
+
+/// Yields execution back to the async runtime.
+pub fn yield_now() -> impl Future<Output = ()> {
+    YieldFuture::default()
+}
+
+#[must_use = "yield_now does nothing unless polled/`await`-ed"]
+#[derive(Default)]
+struct YieldFuture {
+    flag: bool,
+}
+
+impl Future for YieldFuture {
+    type Output = ();
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
+        if self.flag {
+            Poll::Ready(())
+        } else {
+            self.flag = true;
+            cx.waker().clone().wake();
+            Poll::Pending
+        }
+    }
+}

kernel/src/syscall/signal.rs

@@ -110,15 +110,23 @@ impl Syscall<'_> {
         }
         if !set.is_null() {
             let set = set.read()?;
-            info!("rt_sigprocmask: set: {:?}", set);
             const BLOCK: usize = 0;
             const UNBLOCK: usize = 1;
             const SETMASK: usize = 2;
             let mut inner = self.thread.inner.lock();
             match how {
-                BLOCK => inner.sig_mask.add_set(&set),
-                UNBLOCK => inner.sig_mask.remove_set(&set),
-                SETMASK => inner.sig_mask = set,
+                BLOCK => {
+                    info!("rt_sigprocmask: block: {:x?}", set);
+                    inner.sig_mask.add_set(&set);
+                }
+                UNBLOCK => {
+                    info!("rt_sigprocmask: unblock: {:x?}", set);
+                    inner.sig_mask.remove_set(&set)
+                }
+                SETMASK => {
+                    info!("rt_sigprocmask: set: {:x?}", set);
+                    inner.sig_mask = set;
+                }
                 _ => return Err(EINVAL),
             }
         }