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    Subject[PATCH] memcg: fix cgroup rmdir hang v4
    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.

    - 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)
    {
    - 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);
    }

    /*


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