On 2019/10/9 上午1:43, Nikolay Borisov wrote:
> This function is used only during the final phase of freespace cache
> writeout. This is necessary since using the plain btrfs_join_transaction
> api is deadlock prone. The deadlock looks like:
>
> T1:
> btrfs_commit_Transaction
> commit_cowonly_roots
> btrfs_write_dirty_block_groups
> btrfs_wait_cache_io
> __btrfs_wait_cache_io
> btrfs_wait_ordered_range <-- Triggers ordered IO for freespace
> inode and blocks transaction commit until freespace cache
> writeout.
>
> T2: <-- after T1 has triggered the writeout
> finish_ordered_fn
> btrfs_finish_ordered_io
> btrfs_join_transaction <--- this would block waiting for current
> transaction to commit, but since trans commit is waiting for this
> writeout to finish.
>
> The special purpose functions prevents it by simply skipping the "wait
> for writeout" since it's guaranteed the transaction won't proceed until
> we are done.
>
> Signed-off-by: Nikolay Borisov <nbori...@suse.com>
Reviewed-by: Qu Wenruo <w...@suse.com>
Indeed the old _nolock() is pretty confusing. So the rename makes a lot
of sense.
Thanks,
Qu
> ---
> fs/btrfs/inode.c | 12 ++++++------
> fs/btrfs/transaction.c | 2 +-
> fs/btrfs/transaction.h | 2 +-
> 3 files changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 16415815217c..1c6f7c0a2b1d 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -3053,7 +3053,7 @@ static int btrfs_finish_ordered_io(struct
> btrfs_ordered_extent *ordered_extent)
> int compress_type = 0;
> int ret = 0;
> u64 logical_len = ordered_extent->len;
> - bool nolock;
> + bool freespace_inode;
> bool truncated = false;
> bool range_locked = false;
> bool clear_new_delalloc_bytes = false;
> @@ -3064,7 +3064,7 @@ static int btrfs_finish_ordered_io(struct
> btrfs_ordered_extent *ordered_extent)
> !test_bit(BTRFS_ORDERED_DIRECT, &ordered_extent->flags))
> clear_new_delalloc_bytes = true;
>
> - nolock = btrfs_is_free_space_inode(BTRFS_I(inode));
> + freespace_inode = btrfs_is_free_space_inode(BTRFS_I(inode));
>
> if (test_bit(BTRFS_ORDERED_IOERR, &ordered_extent->flags)) {
> ret = -EIO;
> @@ -3095,8 +3095,8 @@ static int btrfs_finish_ordered_io(struct
> btrfs_ordered_extent *ordered_extent)
> btrfs_qgroup_free_data(inode, NULL, ordered_extent->file_offset,
> ordered_extent->len);
> btrfs_ordered_update_i_size(inode, 0, ordered_extent);
> - if (nolock)
> - trans = btrfs_join_transaction_nolock(root);
> + if (freespace_inode)
> + trans = btrfs_join_transaction_spacecache(root);
> else
> trans = btrfs_join_transaction(root);
> if (IS_ERR(trans)) {
> @@ -3130,8 +3130,8 @@ static int btrfs_finish_ordered_io(struct
> btrfs_ordered_extent *ordered_extent)
> EXTENT_DEFRAG, 0, 0, &cached_state);
> }
>
> - if (nolock)
> - trans = btrfs_join_transaction_nolock(root);
> + if (freespace_inode)
> + trans = btrfs_join_transaction_spacecache(root);
> else
> trans = btrfs_join_transaction(root);
> if (IS_ERR(trans)) {
> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
> index 54b8718054ce..6f133906c862 100644
> --- a/fs/btrfs/transaction.c
> +++ b/fs/btrfs/transaction.c
> @@ -729,7 +729,7 @@ struct btrfs_trans_handle *btrfs_join_transaction(struct
> btrfs_root *root)
> true);
> }
>
> -struct btrfs_trans_handle *btrfs_join_transaction_nolock(struct btrfs_root
> *root)
> +struct btrfs_trans_handle *btrfs_join_transaction_spacecache(struct
> btrfs_root *root)
> {
> return start_transaction(root, 0, TRANS_JOIN_NOLOCK,
> BTRFS_RESERVE_NO_FLUSH, true);
> diff --git a/fs/btrfs/transaction.h b/fs/btrfs/transaction.h
> index 2ac89fb0d709..49f7196368f5 100644
> --- a/fs/btrfs/transaction.h
> +++ b/fs/btrfs/transaction.h
> @@ -183,7 +183,7 @@ struct btrfs_trans_handle
> *btrfs_start_transaction_fallback_global_rsv(
> unsigned int num_items,
> int min_factor);
> struct btrfs_trans_handle *btrfs_join_transaction(struct btrfs_root *root);
> -struct btrfs_trans_handle *btrfs_join_transaction_nolock(struct btrfs_root
> *root);
> +struct btrfs_trans_handle *btrfs_join_transaction_spacecache(struct
> btrfs_root *root);
> struct btrfs_trans_handle *btrfs_join_transaction_nostart(struct btrfs_root
> *root);
> struct btrfs_trans_handle *btrfs_attach_transaction(struct btrfs_root *root);
> struct btrfs_trans_handle *btrfs_attach_transaction_barrier(
>
