btrfs: allow device add if balance is paused

Currently paused balance precludes adding a device since they are both
considered exclusive ops and we can have at most one running at a time.
This is problematic in case a filesystem encounters an ENOSPC situation
while balance is running, in this case the only thing the user can do
is mount the fs with "skip_balance" which pauses balance and delete some
data to free up space for balance. However, it should be possible to add
a new device when balance is paused.

Fix this by allowing device add to proceed when balance is paused.
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

@@ -3174,13 +3174,25 @@ static int btrfs_ioctl_defrag(struct file *file, void __user *argp)
 static long btrfs_ioctl_add_dev(struct btrfs_fs_info *fs_info, void __user *arg)
 {
 	struct btrfs_ioctl_vol_args *vol_args;
+	bool restore_op = false;
 	int ret;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
-	if (!btrfs_exclop_start(fs_info, BTRFS_EXCLOP_DEV_ADD))
-		return BTRFS_ERROR_DEV_EXCL_RUN_IN_PROGRESS;
+	if (!btrfs_exclop_start(fs_info, BTRFS_EXCLOP_DEV_ADD)) {
+		if (!btrfs_exclop_start_try_lock(fs_info, BTRFS_EXCLOP_DEV_ADD))
+			return BTRFS_ERROR_DEV_EXCL_RUN_IN_PROGRESS;
+
+		/*
+		 * We can do the device add because we have a paused balanced,
+		 * change the exclusive op type and remember we should bring
+		 * back the paused balance
+		 */
+		fs_info->exclusive_operation = BTRFS_EXCLOP_DEV_ADD;
+		btrfs_exclop_start_unlock(fs_info);
+		restore_op = true;
+	}
 
 	vol_args = memdup_user(arg, sizeof(*vol_args));
 	if (IS_ERR(vol_args)) {
@@ -3196,7 +3208,10 @@ static long btrfs_ioctl_add_dev(struct btrfs_fs_info *fs_info, void __user *arg)
 
 	kfree(vol_args);
 out:
-	btrfs_exclop_finish(fs_info);
+	if (restore_op)
+		btrfs_exclop_balance(fs_info, BTRFS_EXCLOP_BALANCE_PAUSED);
+	else
+		btrfs_exclop_finish(fs_info);
 	return ret;
 }