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>
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@ -4346,6 +4346,10 @@ void btrfs_cleanup_dirty_bgs(struct btrfs_transaction *cur_trans,
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}
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spin_unlock(&cur_trans->dirty_bgs_lock);
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/*
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* Refer to the definition of io_bgs member for details why it's safe
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* to use it without any locking
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*/
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while (!list_empty(&cur_trans->io_bgs)) {
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cache = list_first_entry(&cur_trans->io_bgs,
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struct btrfs_block_group_cache,
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@ -3741,8 +3741,9 @@ again:
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should_put = 0;
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/*
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* the cache_write_mutex is protecting
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* the io_list
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* The cache_write_mutex is protecting the
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* io_list, also refer to the definition of
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* btrfs_transaction::io_bgs for more details
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*/
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list_add_tail(&cache->io_list, io);
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} else {
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@ -3934,6 +3935,10 @@ int btrfs_write_dirty_block_groups(struct btrfs_trans_handle *trans,
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}
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spin_unlock(&cur_trans->dirty_bgs_lock);
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/*
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* Refer to the definition of io_bgs member for details why it's safe
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* to use it without any locking
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*/
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while (!list_empty(io)) {
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cache = list_first_entry(io, struct btrfs_block_group_cache,
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io_list);
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@ -69,6 +69,22 @@ struct btrfs_transaction {
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struct list_head pending_chunks;
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struct list_head switch_commits;
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struct list_head dirty_bgs;
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/*
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* There is no explicit lock which protects io_bgs, rather its
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* consistency is implied by the fact that all the sites which modify
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* it do so under some form of transaction critical section, namely:
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*
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* - btrfs_start_dirty_block_groups - This function can only ever be
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* run by one of the transaction committers. Refer to
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* BTRFS_TRANS_DIRTY_BG_RUN usage in btrfs_commit_transaction
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*
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* - btrfs_write_dirty_blockgroups - this is called by
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* commit_cowonly_roots from transaction critical section
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* (TRANS_STATE_COMMIT_DOING)
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*
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* - btrfs_cleanup_dirty_bgs - called on transaction abort
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*/
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struct list_head io_bgs;
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struct list_head dropped_roots;
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