Fix a bug in SubmissionQueue::sync()

The uring driver may update queue.head, so SubmissionQueue::sync()
should read queue.head instead of queue.tail.

Signed-off-by: Liu Jiang <gerry@linux.alibaba.com>

io-uring-test/src/tests/queue.rs

@@ -12,7 +12,9 @@ pub fn test_nop(ring: &mut IoUring, test: &Test) -> anyhow::Result<()> {
 
     unsafe {
         let mut queue = ring.submission();
+        assert_eq!(queue.len(), 0);
         queue.push(&nop_e).expect("queue is full");
+        assert_eq!(queue.len(), 1);
     }
 
     ring.submit_and_wait(1)?;
@@ -23,6 +25,12 @@ pub fn test_nop(ring: &mut IoUring, test: &Test) -> anyhow::Result<()> {
     assert_eq!(cqes[0].user_data(), 0x42);
     assert_eq!(cqes[0].result(), 0);
 
+    unsafe {
+        let mut queue = ring.submission();
+        queue.sync();
+        assert_eq!(queue.len(), 0);
+    }
+
     Ok(())
 }
 

src/squeue.rs

@@ -178,6 +178,7 @@ impl SubmissionQueue<'_> {
         unsafe {
             (*self.queue.tail).store(self.tail, atomic::Ordering::Release);
             self.tail = (*self.queue.tail).load(atomic::Ordering::Acquire);
+            //self.head = (*self.queue.head).load(atomic::Ordering::Acquire);
         }
     }