On 2018-9-21 11:43, Gao Xiang wrote:
> When the managed cache is enabled, the last reference count
> of a workgroup must be used for its workstation.
>
> Otherwise, it could lead to incorrect (un)freezes in
> the reclaim path, and it would be harmful.
Could you add related race condition stack to demonstrate this problem more
clearly?
Thanks,
>
> Signed-off-by: Gao Xiang <gaoxian...@huawei.com>
> ---
> drivers/staging/erofs/utils.c | 131
> ++++++++++++++++++++++++++++++------------
> 1 file changed, 95 insertions(+), 36 deletions(-)
>
> diff --git a/drivers/staging/erofs/utils.c b/drivers/staging/erofs/utils.c
> index ddd220a..8ef13c8 100644
> --- a/drivers/staging/erofs/utils.c
> +++ b/drivers/staging/erofs/utils.c
> @@ -87,12 +87,28 @@ int erofs_register_workgroup(struct super_block *sb,
> grp = (void *)((unsigned long)grp |
> 1UL << RADIX_TREE_EXCEPTIONAL_SHIFT);
>
> + /*
> + * If managed cache is enabled, the reclaim path assumes
> + * that the last reference count is used for its workstation.
> + * Therefore we should bump up reference count before
> + * making this workgroup visible to other users.
> + */
> +#ifdef EROFS_FS_HAS_MANAGED_CACHE
> + /* refcount should be at least 2 to get on well with reclaim path */
> + __erofs_workgroup_get(grp);
> +#endif
> +
> err = radix_tree_insert(&sbi->workstn_tree,
> grp->index, grp);
>
> - if (!err) {
> +#ifdef EROFS_FS_HAS_MANAGED_CACHE
> + if (unlikely(err))
> + /* it is safe to decrease for refcount >= 2 */
> + atomic_dec(&grp->refcount);
> +#else
> + if (!err)
> __erofs_workgroup_get(grp);
> - }
> +#endif
>
> erofs_workstn_unlock(sbi);
> radix_tree_preload_end();
> @@ -101,19 +117,90 @@ int erofs_register_workgroup(struct super_block *sb,
>
> extern void erofs_workgroup_free_rcu(struct erofs_workgroup *grp);
>
> +static void __erofs_workgroup_free(struct erofs_workgroup *grp)
> +{
> + atomic_long_dec(&erofs_global_shrink_cnt);
> + erofs_workgroup_free_rcu(grp);
> +}
> +
> int erofs_workgroup_put(struct erofs_workgroup *grp)
> {
> int count = atomic_dec_return(&grp->refcount);
>
> if (count == 1)
> atomic_long_inc(&erofs_global_shrink_cnt);
> - else if (!count) {
> - atomic_long_dec(&erofs_global_shrink_cnt);
> - erofs_workgroup_free_rcu(grp);
> - }
> + else if (!count)
> + __erofs_workgroup_free(grp);
> return count;
> }
>
> +#ifdef EROFS_FS_HAS_MANAGED_CACHE
> +
> +static void erofs_workgroup_unfreeze_final(struct erofs_workgroup *grp)
> +{
> + erofs_workgroup_unfreeze(grp, 0);
> + __erofs_workgroup_free(grp);
> +}
> +
> +bool erofs_try_to_release_workgroup(struct erofs_sb_info *sbi,
> + struct erofs_workgroup *grp,
> + bool cleanup)
> +{
> + /*
> + * for managed cache enabled, the refcount of workgroups
> + * themselves could be < 0 (freezed). So there is no guarantee
> + * that all refcount > 0 if managed cache is enabled.
> + */
> + if (!erofs_workgroup_try_to_freeze(grp, 1))
> + return false;
> +
> + /*
> + * note that all cached pages should be unlinked
> + * before delete it from the radix tree.
> + * Otherwise some cached pages of an orphan old workgroup
> + * could be still linked after the new one is available.
> + */
> + if (erofs_try_to_free_all_cached_pages(sbi, grp)) {
> + erofs_workgroup_unfreeze(grp, 1);
> + return false;
> + }
> +
> + /* it is impossible to fail after we freeze the workgroup */
> + if (radix_tree_delete(&sbi->workstn_tree, grp->index) != grp)
> + BUG(); /* should never happen */
> +
> + /*
> + * if managed cache is enable, the last refcount
> + * should indicate the related workstation.
> + */
> + erofs_workgroup_unfreeze_final(grp);
> + return true;
> +}
> +
> +#else
> +
> +bool erofs_try_to_release_workgroup(struct erofs_sb_info *sbi,
> + struct erofs_workgroup *grp,
> + bool cleanup)
> +{
> + int cnt = atomic_read(&grp->refcount);
> +
> + DBG_BUGON(cnt <= 0);
> + DBG_BUGON(cleanup && cnt != 1);
> +
> + if (cnt > 1)
> + return false;
> +
> + if (radix_tree_delete(&sbi->workstn_tree, grp->index) != grp)
> + return false;
> +
> + /* (rarely) could be grabbed again when freeing */
> + erofs_workgroup_put(grp);
> + return true;
> +}
> +
> +#endif
> +
> unsigned long erofs_shrink_workstation(struct erofs_sb_info *sbi,
> unsigned long nr_shrink,
> bool cleanup)
> @@ -130,43 +217,15 @@ unsigned long erofs_shrink_workstation(struct
> erofs_sb_info *sbi,
> batch, first_index, PAGEVEC_SIZE);
>
> for (i = 0; i < found; ++i) {
> - int cnt;
> struct erofs_workgroup *grp = (void *)
> ((unsigned long)batch[i] &
> ~RADIX_TREE_EXCEPTIONAL_ENTRY);
>
> first_index = grp->index + 1;
>
> - cnt = atomic_read(&grp->refcount);
> - BUG_ON(cnt <= 0);
> -
> - if (cleanup)
> - BUG_ON(cnt != 1);
> -
> -#ifndef EROFS_FS_HAS_MANAGED_CACHE
> - else if (cnt > 1)
> -#else
> - if (!erofs_workgroup_try_to_freeze(grp, 1))
> -#endif
> - continue;
> -
> - if (radix_tree_delete(&sbi->workstn_tree,
> - grp->index) != grp) {
> -#ifdef EROFS_FS_HAS_MANAGED_CACHE
> -skip:
> - erofs_workgroup_unfreeze(grp, 1);
> -#endif
> + /* try to shrink each workgroup */
> + if (!erofs_try_to_release_workgroup(sbi, grp, cleanup))
> continue;
> - }
> -
> -#ifdef EROFS_FS_HAS_MANAGED_CACHE
> - if (erofs_try_to_free_all_cached_pages(sbi, grp))
> - goto skip;
> -
> - erofs_workgroup_unfreeze(grp, 1);
> -#endif
> - /* (rarely) grabbed again when freeing */
> - erofs_workgroup_put(grp);
>
> ++freed;
> if (unlikely(!--nr_shrink))
>
