btrfs: avoid blocking open_ctree from cleaner_kthread

This fixes a problem introduced in commit 2f3165ecf1
"btrfs: don't force mounts to wait for cleaner_kthread to delete one or more subvolumes".

open_ctree eventually calls btrfs_replay_log which in turn calls
btrfs_commit_super which tries to lock the cleaner_mutex, causing a
recursive mutex deadlock during mount.

Instead of playing whack-a-mole trying to keep up with all the
functions that may want to lock cleaner_mutex, put all the cleaner_mutex
lockers back where they were, and attack the problem more directly:
keep cleaner_kthread asleep until the filesystem is mounted.

When filesystems are mounted read-only and later remounted read-write,
open_ctree did not set fs_info->open and neither does anything else.
Set this flag in btrfs_remount so that neither btrfs_delete_unused_bgs
nor cleaner_kthread get confused by the common case of "/" filesystem
read-only mount followed by read-write remount.

Signed-off-by: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -1806,6 +1806,13 @@ static int cleaner_kthread(void *arg)
 		if (btrfs_need_cleaner_sleep(root))
 			goto sleep;
 
+		/*
+		 * Do not do anything if we might cause open_ctree() to block
+		 * before we have finished mounting the filesystem.
+		 */
+		if (!root->fs_info->open)
+			goto sleep;
+
 		if (!mutex_trylock(&root->fs_info->cleaner_mutex))
 			goto sleep;
 
@@ -2520,7 +2527,6 @@ int open_ctree(struct super_block *sb,
 	int num_backups_tried = 0;
 	int backup_index = 0;
 	int max_active;
-	bool cleaner_mutex_locked = false;
 
 	tree_root = fs_info->tree_root = btrfs_alloc_root(fs_info, GFP_KERNEL);
 	chunk_root = fs_info->chunk_root = btrfs_alloc_root(fs_info, GFP_KERNEL);
@@ -2999,13 +3005,6 @@ retry_root_backup:
 		goto fail_sysfs;
 	}
 
-	/*
-	 * Hold the cleaner_mutex thread here so that we don't block
-	 * for a long time on btrfs_recover_relocation.  cleaner_kthread
-	 * will wait for us to finish mounting the filesystem.
-	 */
-	mutex_lock(&fs_info->cleaner_mutex);
-	cleaner_mutex_locked = true;
 	fs_info->cleaner_kthread = kthread_run(cleaner_kthread, tree_root,
 					       "btrfs-cleaner");
 	if (IS_ERR(fs_info->cleaner_kthread))
@@ -3065,8 +3064,10 @@ retry_root_backup:
 		ret = btrfs_cleanup_fs_roots(fs_info);
 		if (ret)
 			goto fail_qgroup;
-		/* We locked cleaner_mutex before creating cleaner_kthread. */
+
+		mutex_lock(&fs_info->cleaner_mutex);
 		ret = btrfs_recover_relocation(tree_root);
+		mutex_unlock(&fs_info->cleaner_mutex);
 		if (ret < 0) {
 			btrfs_warn(fs_info, "failed to recover relocation: %d",
 					ret);
@@ -3074,8 +3075,6 @@ retry_root_backup:
 			goto fail_qgroup;
 		}
 	}
-	mutex_unlock(&fs_info->cleaner_mutex);
-	cleaner_mutex_locked = false;
 
 	location.objectid = BTRFS_FS_TREE_OBJECTID;
 	location.type = BTRFS_ROOT_ITEM_KEY;
@@ -3189,10 +3188,6 @@ fail_cleaner:
 	filemap_write_and_wait(fs_info->btree_inode->i_mapping);
 
 fail_sysfs:
-	if (cleaner_mutex_locked) {
-		mutex_unlock(&fs_info->cleaner_mutex);
-		cleaner_mutex_locked = false;
-	}
 	btrfs_sysfs_remove_mounted(fs_info);
 
 fail_fsdev_sysfs:

fs/btrfs/super.c

@@ -1807,6 +1807,8 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
 			}
 		}
 		sb->s_flags &= ~MS_RDONLY;
+
+		fs_info->open = 1;
 	}
 out:
 	wake_up_process(fs_info->transaction_kthread);