1 files changed, 30 insertions, 3 deletions

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 55f3ba6a831c..0493272a7668 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -3758,15 +3758,43 @@ static long btrfs_ioctl_quota_ctl(struct file *file, void __user *arg)
 		goto drop_write;
 	}
 
-	down_write(&fs_info->subvol_sem);
-
 	switch (sa->cmd) {
 	case BTRFS_QUOTA_CTL_ENABLE:
 	case BTRFS_QUOTA_CTL_ENABLE_SIMPLE_QUOTA:
+		down_write(&fs_info->subvol_sem);
 		ret = btrfs_quota_enable(fs_info, sa);
+		up_write(&fs_info->subvol_sem);
 		break;
 	case BTRFS_QUOTA_CTL_DISABLE:
+		/*
+		 * Lock the cleaner mutex to prevent races with concurrent
+		 * relocation, because relocation may be building backrefs for
+		 * blocks of the quota root while we are deleting the root. This
+		 * is like dropping fs roots of deleted snapshots/subvolumes, we
+		 * need the same protection.
+		 *
+		 * This also prevents races between concurrent tasks trying to
+		 * disable quotas, because we will unlock and relock
+		 * qgroup_ioctl_lock across BTRFS_FS_QUOTA_ENABLED changes.
+		 *
+		 * We take this here because we have the dependency of
+		 *
+		 * inode_lock -> subvol_sem
+		 *
+		 * because of rename.  With relocation we can prealloc extents,
+		 * so that makes the dependency chain
+		 *
+		 * cleaner_mutex -> inode_lock -> subvol_sem
+		 *
+		 * so we must take the cleaner_mutex here before we take the
+		 * subvol_sem.  The deadlock can't actually happen, but this
+		 * quiets lockdep.
+		 */
+		mutex_lock(&fs_info->cleaner_mutex);
+		down_write(&fs_info->subvol_sem);
 		ret = btrfs_quota_disable(fs_info);
+		up_write(&fs_info->subvol_sem);
+		mutex_unlock(&fs_info->cleaner_mutex);
 		break;
 	default:
 		ret = -EINVAL;
@@ -3774,7 +3802,6 @@ static long btrfs_ioctl_quota_ctl(struct file *file, void __user *arg)
 	}
 
 	kfree(sa);
-	up_write(&fs_info->subvol_sem);
 drop_write:
 	mnt_drop_write_file(file);
 	return ret;