btrfs: zoned: use dedicated lock for data relocation

Currently, we use btrfs_inode_{lock,unlock}() to grant an exclusive
writeback of the relocation data inode in
btrfs_zoned_data_reloc_{lock,unlock}(). However, that can cause a deadlock
in the following path.

Thread A takes btrfs_inode_lock() and waits for metadata reservation by
e.g, waiting for writeback:

prealloc_file_extent_cluster()
  - btrfs_inode_lock(&inode->vfs_inode, 0);
  - btrfs_prealloc_file_range()
  ...
    - btrfs_replace_file_extents()
      - btrfs_start_transaction
      ...
        - btrfs_reserve_metadata_bytes()

Thread B (e.g, doing a writeback work) needs to wait for the inode lock to
continue writeback process:

do_writepages
  - btrfs_writepages
    - extent_writpages
      - btrfs_zoned_data_reloc_lock(BTRFS_I(inode));
        - btrfs_inode_lock()

The deadlock is caused by relying on the vfs_inode's lock. By using it, we
introduced unnecessary exclusion of writeback and
btrfs_prealloc_file_range(). Also, the lock at this point is useless as we
don't have any dirty pages in the inode yet.

Introduce fs_info->zoned_data_reloc_io_lock and use it for the exclusive
writeback.

Fixes: 35156d8 ("btrfs: zoned: only allow one process to add pages to a relocation inode")
CC: stable@vger.kernel.org # 5.16.x: 869f4cd: btrfs: zoned: encapsulate inode locking for zoned relocation
CC: stable@vger.kernel.org # 5.16.x
CC: stable@vger.kernel.org # 5.17
Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: naota

fs/btrfs/ctree.h

@@ -1060,6 +1060,7 @@ struct btrfs_fs_info {
 	 */
 	spinlock_t relocation_bg_lock;
 	u64 data_reloc_bg;
+	struct mutex zoned_data_reloc_io_lock;
 
 	u64 nr_global_roots;
 

fs/btrfs/disk-io.c

@@ -3156,6 +3156,7 @@ void btrfs_init_fs_info(struct btrfs_fs_info *fs_info)
 	mutex_init(&fs_info->reloc_mutex);
 	mutex_init(&fs_info->delalloc_root_mutex);
 	mutex_init(&fs_info->zoned_meta_io_lock);
+	mutex_init(&fs_info->zoned_data_reloc_io_lock);
 	seqlock_init(&fs_info->profiles_lock);
 
 	INIT_LIST_HEAD(&fs_info->dirty_cowonly_roots);

fs/btrfs/zoned.h

@@ -359,15 +359,15 @@ static inline void btrfs_zoned_data_reloc_lock(struct btrfs_inode *inode)
 	struct btrfs_root *root = inode->root;
 
 	if (btrfs_is_data_reloc_root(root) && btrfs_is_zoned(root->fs_info))
-		btrfs_inode_lock(&inode->vfs_inode, 0);
+		mutex_lock(&root->fs_info->zoned_data_reloc_io_lock);
 }
 
 static inline void btrfs_zoned_data_reloc_unlock(struct btrfs_inode *inode)
 {
 	struct btrfs_root *root = inode->root;
 
 	if (btrfs_is_data_reloc_root(root) && btrfs_is_zoned(root->fs_info))
-		btrfs_inode_unlock(&inode->vfs_inode, 0);
+		mutex_unlock(&root->fs_info->zoned_data_reloc_io_lock);
 }
 
 #endif