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    SubjectRe: [PATCH] memcg: fix cgroup rmdir hang v4
    From
    On Tue, Jun 30, 2009 at 6:47 PM, KAMEZAWA
    Hiroyuki<kamezawa.hiroyu@jp.fujitsu.com> wrote:
    > 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>

    Reviewed-by: Paul Menage <menage@google.com>

    Looks great, thanks.

    Paul

    > ---
    >  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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