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SubjectRe: [PATCH] memcg: fix cgroup rmdir hang v4
* KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-07-01 10:47:47]:

> ok, here.
>
> ==
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>
> After commit: cgroup: fix frequent -EBUSY at rmdir
> ec64f51545fffbc4cb968f0cea56341a4b07e85a
> cgroup's rmdir (especially against memcg) doesn't return -EBUSY
> by temporal ref counts. That commit expects all refs after pre_destroy()
> is temporary but...it wasn't. Then, rmdir can wait permanently.
> This patch tries to fix that and change followings.
>

Sorry for the late review, a few comments below

> - set CGRP_WAIT_ON_RMDIR flag before pre_destroy().
> - clear CGRP_WAIT_ON_RMDIR flag when the subsys finds racy case.
> if there are sleeping ones, wakes them up.
> - rmdir() sleeps only when CGRP_WAIT_ON_RMDIR flag is set.
>
> Changelog v4->v5:
> - added cgroup_exclude_rmdir(), cgroup_release_rmdir().
>
> Changelog v3->v4:
> - rewrite/add comments.
> - remane cgroup_wakeup_rmdir_waiters() to cgroup_wakeup_rmdir_waiter().
> Changelog v2->v3:
> - removed retry_rmdir() callback.
> - make use of CGRP_WAIT_ON_RMDIR flag more.
>
> Reported-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
> include/linux/cgroup.h | 14 ++++++++++++
> kernel/cgroup.c | 55 +++++++++++++++++++++++++++++++++----------------
> mm/memcontrol.c | 23 +++++++++++++++++---
> 3 files changed, 72 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
> @@ -366,6 +366,20 @@ int cgroup_task_count(const struct cgrou
> int cgroup_is_descendant(const struct cgroup *cgrp, struct task_struct *task);
>
> /*
> + * 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_exclude_rmdir(struct cgroup_subsys_state *css);
> +void cgroup_release_rmdir(struct cgroup_subsys_state *css);
> +
> +/*
> * Control Group subsystem type.
> * See Documentation/cgroups/cgroups.txt for details
> */
> 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
> @@ -734,16 +734,28 @@ static void cgroup_d_remove_dir(struct d
> * reference to css->refcnt. In general, this refcnt is expected to goes down
> * to zero, soon.
> *
> - * CGRP_WAIT_ON_RMDIR flag is modified under cgroup's inode->i_mutex;
> + * CGRP_WAIT_ON_RMDIR flag is set under cgroup's inode->i_mutex;
> */
> DECLARE_WAIT_QUEUE_HEAD(cgroup_rmdir_waitq);
>
> -static void cgroup_wakeup_rmdir_waiters(const struct cgroup *cgrp)
> +static void cgroup_wakeup_rmdir_waiter(struct cgroup *cgrp)

Should the function explictly mention rmdir? Also something like
release_rmdir should be called release_and_wakeup to make the action
clearer.

Looks good to me otherwise and clean.

> {
> - if (unlikely(test_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags)))
> + 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)
> {
> @@ -1357,7 +1369,7 @@ int cgroup_attach_task(struct cgroup *cg
> * wake up rmdir() waiter. the rmdir should fail since the cgroup
> * is no longer empty.
> */
> - cgroup_wakeup_rmdir_waiters(cgrp);
> + cgroup_wakeup_rmdir_waiter(cgrp);
> return 0;
> }
>
> @@ -2696,33 +2708,42 @@ again:
> mutex_unlock(&cgroup_mutex);
>
> /*
> + * In general, subsystem has no css->refcnt after pre_destroy(). But
> + * in racy cases, subsystem may have to get css->refcnt after
> + * pre_destroy() and it makes rmdir return with -EBUSY. This sometimes
> + * make rmdir return -EBUSY too often. To avoid that, we use waitqueue
> + * for cgroup's rmdir. CGRP_WAIT_ON_RMDIR is for synchronizing rmdir
> + * and subsystem's reference count handling. Please see css_get/put
> + * and css_tryget() and cgroup_wakeup_rmdir_waiter() implementation.
> + */
> + set_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
> +
> + /*
> * Call pre_destroy handlers of subsys. Notify subsystems
> * that rmdir() request comes.
> */
> ret = cgroup_call_pre_destroy(cgrp);
> - if (ret)
> + if (ret) {
> + clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
> return ret;
> + }
>
> mutex_lock(&cgroup_mutex);
> parent = cgrp->parent;
> if (atomic_read(&cgrp->count) || !list_empty(&cgrp->children)) {
> + clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
> mutex_unlock(&cgroup_mutex);
> return -EBUSY;
> }
> - /*
> - * css_put/get is provided for subsys to grab refcnt to css. In typical
> - * case, subsystem has no reference after pre_destroy(). But, under
> - * hierarchy management, some *temporal* refcnt can be hold.
> - * To avoid returning -EBUSY to a user, waitqueue is used. If subsys
> - * is really busy, it should return -EBUSY at pre_destroy(). wake_up
> - * is called when css_put() is called and refcnt goes down to 0.
> - */
> - set_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
> prepare_to_wait(&cgroup_rmdir_waitq, &wait, TASK_INTERRUPTIBLE);
> -
> if (!cgroup_clear_css_refs(cgrp)) {
> mutex_unlock(&cgroup_mutex);
> - schedule();
> + /*
> + * Because someone may call cgroup_wakeup_rmdir_waiter() before
> + * prepare_to_wait(), we need to check this flag.
> + */
> + if (test_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags))
> + schedule();
> finish_wait(&cgroup_rmdir_waitq, &wait);
> clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
> if (signal_pending(current))
> @@ -3294,7 +3315,7 @@ void __css_put(struct cgroup_subsys_stat
> set_bit(CGRP_RELEASABLE, &cgrp->flags);
> check_for_release(cgrp);
> }
> - cgroup_wakeup_rmdir_waiters(cgrp);
> + cgroup_wakeup_rmdir_waiter(cgrp);
> }
> rcu_read_unlock();
> }
> 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
> @@ -1234,6 +1234,12 @@ static int mem_cgroup_move_account(struc
> ret = 0;
> out:
> unlock_page_cgroup(pc);
> + /*
> + * We charges against "to" which may not have any tasks. Then, "to"
> + * can be under rmdir(). But in current implementation, caller of
> + * this function is just force_empty() and it's garanteed that
> + * "to" is never removed. So, we don't check rmdir status here.
> + */
> return ret;
> }
>
> @@ -1455,6 +1461,7 @@ __mem_cgroup_commit_charge_swapin(struct
> return;
> if (!ptr)
> return;
> + 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);
> @@ -1484,8 +1491,12 @@ __mem_cgroup_commit_charge_swapin(struct
> }
> rcu_read_unlock();
> }
> - /* add this page(page_cgroup) to the LRU we want. */
> -
> + /*
> + * At swapin, we may charge account against cgroup which has no tasks.
> + * 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_release_rmdir(&ptr->css);
> }
>
> void mem_cgroup_commit_charge_swapin(struct page *page, struct mem_cgroup *ptr)
> @@ -1691,7 +1702,7 @@ void mem_cgroup_end_migration(struct mem
>
> if (!mem)
> return;
> -
> + cgroup_exclude_rmdir(&mem->css);
> /* at migration success, oldpage->mapping is NULL. */
> if (oldpage->mapping) {
> target = oldpage;
> @@ -1731,6 +1742,12 @@ void mem_cgroup_end_migration(struct mem
> */
> if (ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
> mem_cgroup_uncharge_page(target);
> + /*
> + * At migration, we may charge account against cgroup which has no tasks
> + * 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_release_rmdir(&mem->css);
> }
>
> /*
>

--
Balbir


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