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btrfs: Document consistency of transaction->io_bgs list

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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>
This commit is contained in:
Nikolay Borisov 2018-02-08 18:25:18 +02:00 committed by David Sterba
parent bf6d7d4900
commit 45ae2c1841
3 changed files with 27 additions and 2 deletions
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4
fs/btrfs/disk-io.c
View File

@ -4346,6 +4346,10 @@ void btrfs_cleanup_dirty_bgs(struct btrfs_transaction *cur_trans,
} }
spin_unlock(&cur_trans->dirty_bgs_lock); 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)) { while (!list_empty(&cur_trans->io_bgs)) {
cache = list_first_entry(&cur_trans->io_bgs, cache = list_first_entry(&cur_trans->io_bgs,
struct btrfs_block_group_cache, struct btrfs_block_group_cache,

9
fs/btrfs/extent-tree.c
View File

@ -3741,8 +3741,9 @@ again:
should_put = 0; should_put = 0;
/* /*
* the cache_write_mutex is protecting * The cache_write_mutex is protecting the
* the io_list * io_list, also refer to the definition of
* btrfs_transaction::io_bgs for more details
*/ */
list_add_tail(&cache->io_list, io); list_add_tail(&cache->io_list, io);
} else { } else {
@ -3934,6 +3935,10 @@ int btrfs_write_dirty_block_groups(struct btrfs_trans_handle *trans,
} }
spin_unlock(&cur_trans->dirty_bgs_lock); 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)) { while (!list_empty(io)) {
cache = list_first_entry(io, struct btrfs_block_group_cache, cache = list_first_entry(io, struct btrfs_block_group_cache,
io_list); io_list);

16
fs/btrfs/transaction.h
View File

@ -69,6 +69,22 @@ struct btrfs_transaction {
struct list_head pending_chunks; struct list_head pending_chunks;
struct list_head switch_commits; struct list_head switch_commits;
struct list_head dirty_bgs; 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 io_bgs;
struct list_head dropped_roots; struct list_head dropped_roots;