btrfs: Document consistency of transaction->io_bgs list

The reason why io_bgs can be modified without holding any lock is
non-obvious. Document it and reference that documentation from the
respective call sites.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -4346,6 +4346,10 @@ void btrfs_cleanup_dirty_bgs(struct btrfs_transaction *cur_trans,
 	}
 	spin_unlock(&cur_trans->dirty_bgs_lock);
 
+	/*
+	 * Refer to the definition of io_bgs member for details why it's safe
+	 * to use it without any locking
+	 */
 	while (!list_empty(&cur_trans->io_bgs)) {
 		cache = list_first_entry(&cur_trans->io_bgs,
 					 struct btrfs_block_group_cache,

fs/btrfs/extent-tree.c

@@ -3741,8 +3741,9 @@ int btrfs_start_dirty_block_groups(struct btrfs_trans_handle *trans)
 				should_put = 0;
 
 				/*
-				 * the cache_write_mutex is protecting
-				 * the io_list
+				 * The cache_write_mutex is protecting the
+				 * io_list, also refer to the definition of
+				 * btrfs_transaction::io_bgs for more details
 				 */
 				list_add_tail(&cache->io_list, io);
 			} else {
@@ -3934,6 +3935,10 @@ int btrfs_write_dirty_block_groups(struct btrfs_trans_handle *trans,
 	}
 	spin_unlock(&cur_trans->dirty_bgs_lock);
 
+	/*
+	 * Refer to the definition of io_bgs member for details why it's safe
+	 * to use it without any locking
+	 */
 	while (!list_empty(io)) {
 		cache = list_first_entry(io, struct btrfs_block_group_cache,
 					 io_list);

fs/btrfs/transaction.h

@@ -69,6 +69,22 @@ struct btrfs_transaction {
 	struct list_head pending_chunks;
 	struct list_head switch_commits;
 	struct list_head dirty_bgs;
+
+	/*
+	 * There is no explicit lock which protects io_bgs, rather its
+	 * consistency is implied by the fact that all the sites which modify
+	 * it do so under some form of transaction critical section, namely:
+	 *
+	 * - btrfs_start_dirty_block_groups - This function can only ever be
+	 *   run by one of the transaction committers. Refer to
+	 *   BTRFS_TRANS_DIRTY_BG_RUN usage in btrfs_commit_transaction
+	 *
+	 * - btrfs_write_dirty_blockgroups - this is called by
+	 *   commit_cowonly_roots from transaction critical section
+	 *   (TRANS_STATE_COMMIT_DOING)
+	 *
+	 * - btrfs_cleanup_dirty_bgs - called on transaction abort
+	 */
 	struct list_head io_bgs;
 	struct list_head dropped_roots;