nfs: fix nonblocking calls to nfs_page_group_lock

nfs_page_group_lock was calling wait_on_bit_lock even when told not to
block. Fix by first trying test_and_set_bit, followed by wait_on_bit_lock
if and only if blocking is allowed.  Return -EAGAIN if nonblocking and the
test_and_set of the bit was already locked.
Signed-off-by: NWeston Andros Adamson <dros@primarydata.com>
Reviewed-by: NPeng Tao <tao.peng@primarydata.com>
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/pagelist.c

@@ -143,23 +143,28 @@ nfs_iocounter_wait(struct nfs_io_counter *c)
  *
  * this lock must be held if modifying the page group list
  *
- * returns result from wait_on_bit_lock: 0 on success, < 0 on error
+ * return 0 on success, < 0 on error: -EDELAY if nonblocking or the
+ * result from wait_on_bit_lock
+ *
+ * NOTE: calling with nonblock=false should always have set the
+ *       lock bit (see fs/buffer.c and other uses of wait_on_bit_lock
+ *       with TASK_UNINTERRUPTIBLE), so there is no need to check the result.
  */
 int
 nfs_page_group_lock(struct nfs_page *req, bool nonblock)
 {
 	struct nfs_page *head = req->wb_head;
-	int ret;
 
 	WARN_ON_ONCE(head != head->wb_head);
 
-	do {
-		ret = wait_on_bit_lock(&head->wb_flags, PG_HEADLOCK,
-			TASK_UNINTERRUPTIBLE);
-	} while (!nonblock && ret != 0);
+	if (!test_and_set_bit(PG_HEADLOCK, &head->wb_flags))
+		return 0;
 
-	WARN_ON_ONCE(ret > 0);
-	return ret;
+	if (!nonblock)
+		return wait_on_bit_lock(&head->wb_flags, PG_HEADLOCK,
+				TASK_UNINTERRUPTIBLE);
+
+	return -EAGAIN;
 }
 
 /*