mailbox: fix race condition

src/mailbox.rs

@@ -58,15 +58,16 @@ impl OneWayMailbox {
             write_volatile(&mut self.pointer, 0);
             write_volatile(&mut self.echo, 0);
         }
+        asm::dmb(); 
     }
 
     // send a pointer from one core to be received by the other core
     pub fn send(&mut self, ptr: usize) -> usize {
         assert!(ptr != 0); // ptr may not be the NULL-like flag        
-        asm::dmb(); // ensure data at (ptr) has been fully written
         unsafe {
             write_volatile(&mut self.pointer, ptr);
         }
+        asm::dmb(); // ensure data at (ptr) has been fully written
         ptr
     }
 
@@ -82,13 +83,13 @@ impl OneWayMailbox {
         ptr
     }
 
-    // return true if and only if the next self.receive() will return
-    // actual data rather than 0
+    // return true if it is guaranteed that the next self.receive()
+    // will return actual data rather than 0
     pub fn available(&self) -> bool {
         let ptr = unsafe {
-            asm::dmb();
             read_volatile(&self.pointer)
         };
+        asm::dmb();
         ptr != 0
     }