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>

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index dcfdb1f..08e33f3 100644 (file)
--- a/fs/btrfs/disk-io.c
+++ b/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,

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 7b4d678..9a67786 100644 (file)
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -3741,8 +3741,9 @@ again:
                                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);

diff --git a/fs/btrfs/transaction.h b/fs/btrfs/transaction.h
index 817fd7c..2762c8d 100644 (file)
--- a/fs/btrfs/transaction.h
+++ b/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;