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    Subject[PATCH 11/23] cgroup: add cgroup->dummy_css
    Date
    cgroup subsystem API is being converted to use css
    (cgroup_subsys_state) as the main handle, which makes things a bit
    awkward for subsystem agnostic core features - the "cgroup.*"
    interface files and various iterations - a bit awkward as they don't
    have a css to use.

    This patch adds cgroup->dummy_css which has NULL ->ss and whose only
    role is pointing back to the cgroup. This will be used to support
    subsystem agnostic features on the coming css based API.

    css_parent() is updated to handle dummy_css's. Note that css will
    soon grow its own ->parent field and css_parent() will be made
    trivial.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    ---
    include/linux/cgroup.h | 11 ++++++++++-
    kernel/cgroup.c | 9 +++++----
    2 files changed, 15 insertions(+), 5 deletions(-)

    diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
    index 2e0e9e2..085ca93 100644
    --- a/include/linux/cgroup.h
    +++ b/include/linux/cgroup.h
    @@ -225,6 +225,9 @@ struct cgroup {
    struct list_head pidlists;
    struct mutex pidlist_mutex;

    + /* dummy css with NULL ->ss, points back to this cgroup */
    + struct cgroup_subsys_state dummy_css;
    +
    /* For css percpu_ref killing and RCU-protected deletion */
    struct rcu_head rcu_head;
    struct work_struct destroy_work;
    @@ -669,7 +672,13 @@ struct cgroup_subsys_state *css_parent(struct cgroup_subsys_state *css)
    {
    struct cgroup *parent_cgrp = css->cgroup->parent;

    - return parent_cgrp ? parent_cgrp->subsys[css->ss->subsys_id] : NULL;
    + if (!parent_cgrp)
    + return NULL;
    +
    + if (css->ss)
    + return parent_cgrp->subsys[css->ss->subsys_id];
    + else
    + return &parent_cgrp->dummy_css;
    }

    /**
    diff --git a/kernel/cgroup.c b/kernel/cgroup.c
    index b413e22..bb87c9f 100644
    --- a/kernel/cgroup.c
    +++ b/kernel/cgroup.c
    @@ -1365,6 +1365,7 @@ static void init_cgroup_housekeeping(struct cgroup *cgrp)
    INIT_LIST_HEAD(&cgrp->release_list);
    INIT_LIST_HEAD(&cgrp->pidlists);
    mutex_init(&cgrp->pidlist_mutex);
    + cgrp->dummy_css.cgroup = cgrp;
    INIT_LIST_HEAD(&cgrp->event_list);
    spin_lock_init(&cgrp->event_list_lock);
    simple_xattrs_init(&cgrp->xattrs);
    @@ -2278,7 +2279,7 @@ static struct cgroup_subsys_state *cgroup_file_css(struct cfent *cfe)

    if (cft->ss)
    return cgrp->subsys[cft->ss->subsys_id];
    - return NULL;
    + return &cgrp->dummy_css;
    }

    /* A buffer size big enough for numbers or short strings */
    @@ -2460,7 +2461,7 @@ static int cgroup_file_open(struct inode *inode, struct file *file)
    * unpinned either on open failure or release. This ensures that
    * @css stays alive for all file operations.
    */
    - if (css && !css_tryget(css))
    + if (css->ss && !css_tryget(css))
    return -ENODEV;

    if (cft->read_map || cft->read_seq_string) {
    @@ -2470,7 +2471,7 @@ static int cgroup_file_open(struct inode *inode, struct file *file)
    err = cft->open(inode, file);
    }

    - if (css && err)
    + if (css->ss && err)
    css_put(css);
    return err;
    }
    @@ -2484,7 +2485,7 @@ static int cgroup_file_release(struct inode *inode, struct file *file)

    if (cft->release)
    ret = cft->release(inode, file);
    - if (css)
    + if (css->ss)
    css_put(css);
    return ret;
    }
    --
    1.8.3.1


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