Commit 45ae2c18 authored by Nikolay Borisov's avatar Nikolay Borisov Committed by David Sterba

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: default avatarNikolay Borisov <nborisov@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent bf6d7d49
......@@ -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,
......
......@@ -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);
......
......@@ -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;
......
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