From 45ae2c1841c31c90077cf427c09ea0e83e381026 Mon Sep 17 00:00:00 2001 From: Nikolay Borisov Date: Thu, 8 Feb 2018 18:25:18 +0200 Subject: [PATCH] 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 Signed-off-by: David Sterba --- fs/btrfs/disk-io.c | 4 ++++ fs/btrfs/extent-tree.c | 9 +++++++-- fs/btrfs/transaction.h | 16 ++++++++++++++++ 3 files changed, 27 insertions(+), 2 deletions(-) diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c index dcfdb1f17f1b..08e33f316a86 100644 --- 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 7b4d6789cc66..9a677860f64d 100644 --- 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 817fd7c9836b..2762c8d6191c 100644 --- 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; -- 2.30.2