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    Subject[PATCH 12/17] cgroup: introduce CSS_ONLINE flag and on/offline_css() helpers
    Date
    New helpers on/offline_css() respectively wrap ->post_create() and
    ->pre_destroy() invocations. online_css() sets CSS_ONLINE after
    ->post_create() is complete and offline_css() invokes ->pre_destroy()
    iff CSS_ONLINE is set and clears it while also handling the temporary
    dropping of cgroup_mutex.

    This patch doesn't introduce any behavior change at the moment but
    will be used to improve cgroup_create() failure path and allow
    ->post_create() to fail.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    ---
    include/linux/cgroup.h | 1 +
    kernel/cgroup.c | 65 ++++++++++++++++++++++++++++++++------------------
    2 files changed, 43 insertions(+), 23 deletions(-)

    diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
    index e9f07ef..c487315 100644
    --- a/include/linux/cgroup.h
    +++ b/include/linux/cgroup.h
    @@ -82,6 +82,7 @@ struct cgroup_subsys_state {
    /* bits in struct cgroup_subsys_state flags field */
    enum {
    CSS_ROOT = (1 << 0), /* this CSS is the root of the subsystem */
    + CSS_ONLINE = (1 << 1), /* between ->post_create() and ->pre_destroy() */
    };

    /* Caller must verify that the css is not for root cgroup */
    diff --git a/kernel/cgroup.c b/kernel/cgroup.c
    index afc47d6..c3087f5 100644
    --- a/kernel/cgroup.c
    +++ b/kernel/cgroup.c
    @@ -4035,6 +4035,42 @@ static void init_cgroup_css(struct cgroup_subsys_state *css,
    INIT_WORK(&css->dput_work, css_dput_fn);
    }

    +/* invoke ->post_create() on a new CSS and mark it online */
    +static void online_css(struct cgroup_subsys *ss, struct cgroup *cgrp)
    +{
    + lockdep_assert_held(&cgroup_mutex);
    +
    + if (ss->post_create)
    + ss->post_create(cgrp);
    + cgrp->subsys[ss->subsys_id]->flags |= CSS_ONLINE;
    +}
    +
    +/* if the CSS is online, invoke ->pre_destory() on it and mark it offline */
    +static void offline_css(struct cgroup_subsys *ss, struct cgroup *cgrp)
    + __releases(&cgroup_mutex) __acquires(&cgroup_mutex)
    +{
    + struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
    +
    + lockdep_assert_held(&cgroup_mutex);
    +
    + if (!(css->flags & CSS_ONLINE))
    + return;
    +
    + /*
    + * pre_destroy() should be called with cgroup_mutex unlocked. See
    + * 3fa59dfbc3 ("cgroup: fix potential deadlock in pre_destroy") for
    + * details. This temporary unlocking should go away once
    + * cgroup_mutex is unexported from controllers.
    + */
    + if (ss->pre_destroy) {
    + mutex_unlock(&cgroup_mutex);
    + ss->pre_destroy(cgrp);
    + mutex_lock(&cgroup_mutex);
    + }
    +
    + cgrp->subsys[ss->subsys_id]->flags &= ~CSS_ONLINE;
    +}
    +
    /*
    * cgroup_create - create a cgroup
    * @parent: cgroup that will be parent of the new cgroup
    @@ -4137,8 +4173,7 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
    dget(dentry);

    /* creation succeeded, notify subsystems */
    - if (ss->post_create)
    - ss->post_create(cgrp);
    + online_css(ss, cgrp);
    }

    err = cgroup_populate_dir(cgrp, true, root->subsys_mask);
    @@ -4240,18 +4275,9 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
    }
    set_bit(CGRP_REMOVED, &cgrp->flags);

    - /*
    - * Tell subsystems to initate destruction. pre_destroy() should be
    - * called with cgroup_mutex unlocked. See 3fa59dfbc3 ("cgroup: fix
    - * potential deadlock in pre_destroy") for details. This temporary
    - * unlocking should go away once cgroup_mutex is unexported from
    - * controllers.
    - */
    - mutex_unlock(&cgroup_mutex);
    + /* tell subsystems to initate destruction */
    for_each_subsys(cgrp->root, ss)
    - if (ss->pre_destroy)
    - ss->pre_destroy(cgrp);
    - mutex_lock(&cgroup_mutex);
    + offline_css(ss, cgrp);

    /*
    * Put all the base refs. Each css holds an extra reference to the
    @@ -4354,9 +4380,7 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
    BUG_ON(!list_empty(&init_task.tasks));

    ss->active = 1;
    -
    - if (ss->post_create)
    - ss->post_create(dummytop);
    + online_css(ss, dummytop);

    mutex_unlock(&cgroup_mutex);

    @@ -4469,9 +4493,7 @@ int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss)
    write_unlock(&css_set_lock);

    ss->active = 1;
    -
    - if (ss->post_create)
    - ss->post_create(dummytop);
    + online_css(ss, dummytop);

    /* success! */
    mutex_unlock(&cgroup_mutex);
    @@ -4501,12 +4523,9 @@ void cgroup_unload_subsys(struct cgroup_subsys *ss)
    */
    BUG_ON(ss->root != &rootnode);

    - /* ->pre_destroy() should be called outside cgroup_mutex for now */
    - if (ss->pre_destroy)
    - ss->pre_destroy(dummytop);
    -
    mutex_lock(&cgroup_mutex);

    + offline_css(ss, dummytop);
    ss->active = 0;

    if (ss->use_id) {
    --
    1.7.11.7


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