From 1f4add397b3d6774ea4a685b5b4e7de6ca911f3d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Stein?= <bjoern.stein@quantum-factory.de>
Date: Fri, 30 Aug 2019 15:56:42 +0800
Subject: [PATCH] mailbox: fix race condition

---
 src/mailbox.rs | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/src/mailbox.rs b/src/mailbox.rs
index 1459a2db..a258f292 100644
--- a/src/mailbox.rs
+++ b/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
     }