Add MutexSpin and several syscalls.

os/src/sync/mod.rs

@@ -1,3 +1,5 @@
 mod up;
+mod mutex;
 
-pub use up::UPSafeCell;
+pub use up::UPSafeCell;
+pub use mutex::{Mutex, MutexSpin};

os/src/sync/mutex.rs

@@ -0,0 +1,40 @@
+use super::UPSafeCell;
+use crate::task::suspend_current_and_run_next;
+
+pub trait Mutex: Sync + Send {
+    fn lock(&self);
+    fn unlock(&self);
+}
+
+pub struct MutexSpin {
+    locked: UPSafeCell<bool>,
+}
+
+impl MutexSpin {
+    pub fn new() -> Self {
+        Self {
+            locked: unsafe { UPSafeCell::new(false) },
+        }
+    }
+}
+
+impl Mutex for MutexSpin {
+    fn lock(&self) {
+        loop {
+            let mut locked = self.locked.exclusive_access();
+            if *locked {
+                drop(locked);
+                suspend_current_and_run_next();
+                continue;
+            } else {
+                *locked = true;
+                return;
+            }
+        }
+    }
+
+    fn unlock(&self) {
+        let mut locked = self.locked.exclusive_access();
+        *locked = false;
+    }
+}