On Tue, Dec 03, 2013 at 01:33:39AM +0800, Wang Shilong wrote:
> From: Wang Shilong <wangsl.f...@cn.fujitsu.com>
>
> We came a race condition when scrubbing superblocks, the story is:
>
> In commiting transaction, we will update last_trans_commited after
> writting superblocks. if a scrub start after writting superblocks
> and before last_trans_commited, generation mismatch happens!
>
> We fix it by protecting writting superblock and updating last_trans_commited
> with tree_log_mutex.
>
> Reported-by: Sebastian Ochmann <ochm...@informatik.uni-bonn.de>
> Signed-off-by: Wang Shilong <wangsl.f...@cn.fujitsu.com>
> ---
> Changelog:
> v2->v3:move tree_log_mutex out of device_list_mutex.
> v1->v2: use right way to fix the problem.
> ---
> fs/btrfs/scrub.c | 11 +++++++----
> fs/btrfs/transaction.c | 13 ++++++++++---
> 2 files changed, 17 insertions(+), 7 deletions(-)
>
> diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
> index 561e2f1..a9ed102 100644
> --- a/fs/btrfs/scrub.c
> +++ b/fs/btrfs/scrub.c
> @@ -2887,6 +2887,7 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64
> devid, u64 start,
> }
>
>
> + mutex_lock(&fs_info->tree_log_mutex);
> mutex_lock(&fs_info->fs_devices->device_list_mutex);
> dev = btrfs_find_device(fs_info, devid, NULL, NULL);
> if (!dev || (dev->missing && !is_dev_replace)) {
> @@ -2932,14 +2933,16 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info,
> u64 devid, u64 start,
> atomic_inc(&fs_info->scrubs_running);
> mutex_unlock(&fs_info->scrub_lock);
>
> + /*
> + * holding tree_log_mutex we can avoid generation mismatch while
> + * scrubbing superblocks, see comments in commiting transaction
> + * when updating last_trans_commited.
> + */
> if (!is_dev_replace) {
> - /*
> - * by holding device list mutex, we can
> - * kick off writing super in log tree sync.
> - */
> ret = scrub_supers(sctx, dev);
> }
> mutex_unlock(&fs_info->fs_devices->device_list_mutex);
> + mutex_unlock(&fs_info->tree_log_mutex);
IIRC, we already have btrfs_scrub_{pause, continue}() to avoid race
situations between committing transaction and scrub processes, why not use that
instead?
(Actually I don't like adding another lock unless it's been proved necessary
and correct with lockdep.)
thanks,
-liubo
>
> if (!ret)
> ret = scrub_enumerate_chunks(sctx, dev, start, end,
> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
> index c6a872a..052eb22 100644
> --- a/fs/btrfs/transaction.c
> +++ b/fs/btrfs/transaction.c
> @@ -1898,15 +1898,22 @@ int btrfs_commit_transaction(struct
> btrfs_trans_handle *trans,
> goto cleanup_transaction;
> }
>
> + btrfs_finish_extent_commit(trans, root);
> +
> + /*
> + * we must gurantee last_trans_commited update is protected by
> + * tree_log_mutex with write_ctree_super together, otherwise,
> + * scubbing super will come in before updating last_trans_commited
> + * and we will get generation mismatch when scrubbing superblocks.
> + */
> + root->fs_info->last_trans_committed = cur_trans->transid;
> +
> /*
> * the super is written, we can safely allow the tree-loggers
> * to go about their business
> */
> mutex_unlock(&root->fs_info->tree_log_mutex);
>
> - btrfs_finish_extent_commit(trans, root);
> -
> - root->fs_info->last_trans_committed = cur_trans->transid;
> /*
> * We needn't acquire the lock here because there is no other task
> * which can change it.
> --
> 1.8.4
>
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