btrfs: introduce btrfs_try_lock_balance

This function contains the factored out locking sequence of
btrfs_ioctl_balance. Having this piece of code separate helps to
simplify btrfs_ioctl_balance which has too complicated.  This will be
used in the next patch to streamline the logic in btrfs_ioctl_balance.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -4355,6 +4355,72 @@ void btrfs_update_ioctl_balance_args(struct btrfs_fs_info *fs_info,
 	spin_unlock(&fs_info->balance_lock);
 }
 
+/**
+ * Try to acquire fs_info::balance_mutex as well as set BTRFS_EXLCOP_BALANCE as
+ * required.
+ *
+ * @fs_info:       the filesystem
+ * @excl_acquired: ptr to boolean value which is set to false in case balance
+ *                 is being resumed
+ *
+ * Return 0 on success in which case both fs_info::balance is acquired as well
+ * as exclusive ops are blocked. In case of failure return an error code.
+ */
+static int btrfs_try_lock_balance(struct btrfs_fs_info *fs_info, bool *excl_acquired)
+{
+	int ret;
+
+	/*
+	 * Exclusive operation is locked. Three possibilities:
+	 *   (1) some other op is running
+	 *   (2) balance is running
+	 *   (3) balance is paused -- special case (think resume)
+	 */
+	while (1) {
+		if (btrfs_exclop_start(fs_info, BTRFS_EXCLOP_BALANCE)) {
+			*excl_acquired = true;
+			mutex_lock(&fs_info->balance_mutex);
+			return 0;
+		}
+
+		mutex_lock(&fs_info->balance_mutex);
+		if (fs_info->balance_ctl) {
+			/* This is either (2) or (3) */
+			if (test_bit(BTRFS_FS_BALANCE_RUNNING, &fs_info->flags)) {
+				/* This is (2) */
+				ret = -EINPROGRESS;
+				goto out_failure;
+
+			} else {
+				mutex_unlock(&fs_info->balance_mutex);
+				/*
+				 * Lock released to allow other waiters to
+				 * continue, we'll reexamine the status again.
+				 */
+				mutex_lock(&fs_info->balance_mutex);
+
+				if (fs_info->balance_ctl &&
+				    !test_bit(BTRFS_FS_BALANCE_RUNNING, &fs_info->flags)) {
+					/* This is (3) */
+					*excl_acquired = false;
+					return 0;
+				}
+			}
+		} else {
+			/* This is (1) */
+			ret = BTRFS_ERROR_DEV_EXCL_RUN_IN_PROGRESS;
+			goto out_failure;
+		}
+
+		mutex_unlock(&fs_info->balance_mutex);
+	}
+
+out_failure:
+	mutex_unlock(&fs_info->balance_mutex);
+	*excl_acquired = false;
+	return ret;
+}
+
 static long btrfs_ioctl_balance(struct file *file, void __user *arg)
 {
 	struct btrfs_root *root = BTRFS_I(file_inode(file))->root;