fix the contention test

patch by @pftbest 

src/ring_buffer/mod.rs

@@ -29,6 +29,10 @@ impl AtomicUsize {
         unsafe { &mut *self.v.get() }
     }
 
+    pub fn load_acquire(&self) -> usize {
+        unsafe { intrinsics::atomic_load_acq(self.v.get()) }
+    }
+
     pub fn load_relaxed(&self) -> usize {
         unsafe { intrinsics::atomic_load_relaxed(self.v.get()) }
     }

src/ring_buffer/spsc.rs

@@ -45,7 +45,7 @@ where
         let n = rb.capacity() + 1;
         let buffer: &[T] = unsafe { rb.buffer.as_ref() };
 
-        let tail = rb.tail.load_relaxed();
+        let tail = rb.tail.load_acquire();
         let head = rb.head.load_relaxed();
         if head != tail {
             let item = unsafe { ptr::read(buffer.get_unchecked(head)) };