Re: [PATCH 2/2] cgroup: exlclude release rmdir

[Paul Menage]

On Tue, Jun 30, 2009 at 2:03 AM, KAMEZAWA
Hiroyuki<kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
>
> Paul Menage pointed out that css_get()/put() only for avoiding race with
> rmdir() is complicated and these should be treated as it is for.
>
> This adds
>   - cgroup_exclude_rmdir() ....prevent rmdir() for a while.
>   - cgroup_release_rmdir() ....rerun rmdir() if necessary.
> And hides cgroup_wakeup_rmdir_waiter() into kernel/cgroup.c, again.

Wouldn't it be better to merge these into a single patch? Having one
patch that exposes complexity only to take it away in the following
patch seems unnecessary; the combined patch would be simpler than the
constituents.

Paul

>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
>
> ---
>  include/linux/cgroup.h |   21 +++++++++++----------
>  kernel/cgroup.c        |   17 +++++++++++++++--
>  mm/memcontrol.c        |   12 ++++--------
>  3 files changed, 30 insertions(+), 20 deletions(-)
>
> Index: mmotm-2.6.31-Jun25/include/linux/cgroup.h
> ===================================================================
> --- mmotm-2.6.31-Jun25.orig/include/linux/cgroup.h
> +++ mmotm-2.6.31-Jun25/include/linux/cgroup.h
> @@ -366,17 +366,18 @@ int cgroup_task_count(const struct cgrou
>  int cgroup_is_descendant(const struct cgroup *cgrp, struct task_struct *task);
>
>  /*
> - * Allow to use CGRP_WAIT_ON_RMDIR flag to check race with rmdir() for subsys.
> - * Subsys can call this function if it's necessary to call pre_destroy() again
> - * because it adds not-temporary refs to css after or while pre_destroy().
> - * The caller of this function should use css_tryget(), too.
> + * When the subsys has to access css and may add permanent refcnt to css,
> + * it should take care of racy conditions with rmdir(). Following set of
> + * functions, is for stop/restart rmdir if necessary.
> + * Because these will call css_get/put, "css" should be alive css.
> + *
> + *  cgroup_exclude_rmdir();
> + *  ...do some jobs which may access arbitrary empty cgroup
> + *  cgroup_release_rmdir();
>  */
> -void __cgroup_wakeup_rmdir_waiters(void);
> -static inline void cgroup_wakeup_rmdir_waiter(struct cgroup *cgrp)
> -{
> -       if (unlikely(test_and_clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags)))
> -               __cgroup_wakeup_rmdir_waiters();
> -}
> +
> +void cgroup_exclude_rmdir(struct cgroup_subsys_state *css);
> +void cgroup_release_rmdir(struct cgroup_subsys_state *css);
>
>  /*
>  * Control Group subsystem type.
> Index: mmotm-2.6.31-Jun25/kernel/cgroup.c
> ===================================================================
> --- mmotm-2.6.31-Jun25.orig/kernel/cgroup.c
> +++ mmotm-2.6.31-Jun25/kernel/cgroup.c
> @@ -738,11 +738,24 @@ static void cgroup_d_remove_dir(struct d
>  */
>  DECLARE_WAIT_QUEUE_HEAD(cgroup_rmdir_waitq);
>
> -void __cgroup_wakeup_rmdir_waiters(void)
> +static void cgroup_wakeup_rmdir_waiter(struct cgroup *cgrp)
>  {
> -       wake_up_all(&cgroup_rmdir_waitq);
> +       if (unlikely(test_and_clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags)))
> +               wake_up_all(&cgroup_rmdir_waitq);
>  }
>
> +void cgroup_exclude_rmdir(struct cgroup_subsys_state *css)
> +{
> +       css_get(css);
> +}
> +
> +void cgroup_release_rmdir(struct cgroup_subsys_state *css)
> +{
> +       cgroup_wakeup_rmdir_waiter(css->cgroup);
> +       css_put(css);
> +}
> +
> +
>  static int rebind_subsystems(struct cgroupfs_root *root,
>                              unsigned long final_bits)
>  {
> Index: mmotm-2.6.31-Jun25/mm/memcontrol.c
> ===================================================================
> --- mmotm-2.6.31-Jun25.orig/mm/memcontrol.c
> +++ mmotm-2.6.31-Jun25/mm/memcontrol.c
> @@ -1461,7 +1461,7 @@ __mem_cgroup_commit_charge_swapin(struct
>                return;
>        if (!ptr)
>                return;
> -       css_get(&ptr->css);
> +       cgroup_exclude_rmdir(&ptr->css);
>        pc = lookup_page_cgroup(page);
>        mem_cgroup_lru_del_before_commit_swapcache(page);
>        __mem_cgroup_commit_charge(ptr, pc, ctype);
> @@ -1496,9 +1496,7 @@ __mem_cgroup_commit_charge_swapin(struct
>         * So, rmdir()->pre_destroy() can be called while we do this charge.
>         * In that case, we need to call pre_destroy() again. check it here.
>         */
> -       cgroup_wakeup_rmdir_waiter(ptr->css.cgroup);
> -       css_put(&ptr->css);
> -
> +       cgroup_release_rmdir(&ptr->css);
>  }
>
>  void mem_cgroup_commit_charge_swapin(struct page *page, struct mem_cgroup *ptr)
> @@ -1704,7 +1702,7 @@ void mem_cgroup_end_migration(struct mem
>
>        if (!mem)
>                return;
> -       css_get(&mem->css);
> +       cgroup_exclude_rmdir(&mem->css);
>        /* at migration success, oldpage->mapping is NULL. */
>        if (oldpage->mapping) {
>                target = oldpage;
> @@ -1749,9 +1747,7 @@ void mem_cgroup_end_migration(struct mem
>         * So, rmdir()->pre_destroy() can be called while we do this charge.
>         * In that case, we need to call pre_destroy() again. check it here.
>         */
> -       cgroup_wakeup_rmdir_waiter(mem->css.cgroup);
> -       css_put(&mem->css);
> -
> +       cgroup_release_rmdir(&mem->css);
>  }
>
>  /*
>
>
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