Fixed MPMC panic on wrapping sub

src/mpmc.rs

@@ -481,7 +481,7 @@ unsafe fn dequeue<T>(buffer: *mut Cell<T>, dequeue_pos: &AtomicU8, mask: u8) ->
     loop {
         cell = buffer.add(usize::from(pos & mask));
         let seq = (*cell).sequence.load(Ordering::Acquire);
-        let dif = (seq as i8) - ((pos.wrapping_add(1)) as i8);
+        let dif = (seq as i8).wrapping_sub((pos.wrapping_add(1)) as i8);
 
         if dif == 0 {
             if dequeue_pos
@@ -521,7 +521,7 @@ unsafe fn enqueue<T>(
     loop {
         cell = buffer.add(usize::from(pos & mask));
         let seq = (*cell).sequence.load(Ordering::Acquire);
-        let dif = (seq as i8) - (pos as i8);
+        let dif = (seq as i8).wrapping_sub(pos as i8);
 
         if dif == 0 {
             if enqueue_pos

tests/tsan.rs

@@ -131,6 +131,7 @@ fn mpmc_contention() {
 
             for i in 0..(16 * N) {
                 sum = sum.wrapping_add(i);
+                println!("enqueue {}", i);
                 while let Err(_) = Q.enqueue(i) {}
             }
 
@@ -146,6 +147,7 @@ fn mpmc_contention() {
                     match Q.dequeue() {
                         Some(v) => {
                             sum = sum.wrapping_add(v);
+                            println!("dequeue {}", v);
                             break;
                         }
                         _ => {}