btrfs: zoned: properly take lock to read/update block group's zoned variables

__btrfs_add_free_space_zoned() references and modifies bg's alloc_offset,
ro, and zone_unusable, but without taking the lock. It is mostly safe
because they monotonically increase (at least for now) and this function is
mostly called by a transaction commit, which is serialized by itself.

Still, taking the lock is a safer and correct option and I'm going to add a
change to reset zone_unusable while a block group is still alive. So, add
locking around the operations.

Fixes: 169e0da9 ("btrfs: zoned: track unusable bytes for zones")
CC: stable@vger.kernel.org # 5.15+
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/free-space-cache.c

@@ -2697,15 +2697,16 @@ static int __btrfs_add_free_space_zoned(struct btrfs_block_group *block_group,
 	u64 offset = bytenr - block_group->start;
 	u64 to_free, to_unusable;
 	int bg_reclaim_threshold = 0;
-	bool initial = ((size == block_group->length) && (block_group->alloc_offset == 0));
+	bool initial;
 	u64 reclaimable_unusable;
 
-	WARN_ON(!initial && offset + size > block_group->zone_capacity);
+	spin_lock(&block_group->lock);
 
+	initial = ((size == block_group->length) && (block_group->alloc_offset == 0));
+	WARN_ON(!initial && offset + size > block_group->zone_capacity);
 	if (!initial)
 		bg_reclaim_threshold = READ_ONCE(sinfo->bg_reclaim_threshold);
 
-	spin_lock(&ctl->tree_lock);
 	if (!used)
 		to_free = size;
 	else if (initial)
@@ -2718,7 +2719,9 @@ static int __btrfs_add_free_space_zoned(struct btrfs_block_group *block_group,
 		to_free = offset + size - block_group->alloc_offset;
 	to_unusable = size - to_free;
 
+	spin_lock(&ctl->tree_lock);
 	ctl->free_space += to_free;
+	spin_unlock(&ctl->tree_lock);
 	/*
 	 * If the block group is read-only, we should account freed space into
 	 * bytes_readonly.
@@ -2727,11 +2730,8 @@ static int __btrfs_add_free_space_zoned(struct btrfs_block_group *block_group,
 		block_group->zone_unusable += to_unusable;
 		WARN_ON(block_group->zone_unusable > block_group->length);
 	}
-	spin_unlock(&ctl->tree_lock);
 	if (!used) {
-		spin_lock(&block_group->lock);
 		block_group->alloc_offset -= size;
-		spin_unlock(&block_group->lock);
 	}
 
 	reclaimable_unusable = block_group->zone_unusable -
@@ -2745,6 +2745,8 @@ static int __btrfs_add_free_space_zoned(struct btrfs_block_group *block_group,
 		btrfs_mark_bg_to_reclaim(block_group);
 	}
 
+	spin_unlock(&block_group->lock);
+
 	return 0;
 }