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    Subject[PATCH 06/12] cgroup: factor out cgroup_drain_offline() from cgroup_subtree_control_write()
    Date
    Factor out async css offline draining into cgroup_drain_offline().

    * Nest subsystem walk inside child walk. The child walk will later be
    converted to subtree walk which is a bit more expensive.

    * Relocate the draining above subsystem mask preparation, which
    doesn't create any behavior differences but helps further
    refactoring.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    ---
    kernel/cgroup.c | 77 ++++++++++++++++++++++++++++++++++++++-------------------
    1 file changed, 52 insertions(+), 25 deletions(-)

    diff --git a/kernel/cgroup.c b/kernel/cgroup.c
    index 0282797..7966750 100644
    --- a/kernel/cgroup.c
    +++ b/kernel/cgroup.c
    @@ -2960,6 +2960,53 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
    return ret;
    }

    +/**
    + * cgroup_drain_offline - wait for previously offlined csses to go away
    + * @cgrp: parent of the target cgroups
    + *
    + * Because css offlining is asynchronous, userland may try to re-enable a
    + * controller while the previous css is still around. This function drains
    + * the previous css instances of @cgrp's children.
    + *
    + * Must be called with cgroup_mutex held. Returns %false if there were no
    + * dying css instances. Returns %true if there were one or more and this
    + * function waited. On %true return, cgroup_mutex has been dropped and
    + * re-acquired inbetween which anything could have happened. The caller
    + * typically would have to start over.
    + */
    +static bool cgroup_drain_offline(struct cgroup *cgrp)
    +{
    + struct cgroup *dsct;
    + struct cgroup_subsys *ss;
    + int ssid;
    +
    + lockdep_assert_held(&cgroup_mutex);
    +
    + cgroup_for_each_live_child(dsct, cgrp) {
    + for_each_subsys(ss, ssid) {
    + struct cgroup_subsys_state *css = cgroup_css(dsct, ss);
    + DEFINE_WAIT(wait);
    +
    + if (!css)
    + continue;
    +
    + cgroup_get(dsct);
    + prepare_to_wait(&dsct->offline_waitq, &wait,
    + TASK_UNINTERRUPTIBLE);
    +
    + mutex_unlock(&cgroup_mutex);
    + schedule();
    + finish_wait(&dsct->offline_waitq, &wait);
    + mutex_lock(&cgroup_mutex);
    +
    + cgroup_put(dsct);
    + return true;
    + }
    + }
    +
    + return false;
    +}
    +
    /* change the enabled child controllers for a cgroup in the default hierarchy */
    static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
    char *buf, size_t nbytes,
    @@ -3048,6 +3095,11 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
    goto out_unlock;
    }

    + if (cgroup_drain_offline(cgrp)) {
    + cgroup_kn_unlock(of->kn);
    + return restart_syscall();
    + }
    +
    /*
    * Update subsys masks and calculate what needs to be done. More
    * subsystems than specified may need to be enabled or disabled
    @@ -3063,31 +3115,6 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
    enable |= css_enable;
    disable |= css_disable;

    - /*
    - * Because css offlining is asynchronous, userland might try to
    - * re-enable the same controller while the previous instance is
    - * still around. In such cases, wait till it's gone using
    - * offline_waitq.
    - */
    - do_each_subsys_mask(ss, ssid, css_enable) {
    - cgroup_for_each_live_child(child, cgrp) {
    - DEFINE_WAIT(wait);
    -
    - if (!cgroup_css(child, ss))
    - continue;
    -
    - cgroup_get(child);
    - prepare_to_wait(&child->offline_waitq, &wait,
    - TASK_UNINTERRUPTIBLE);
    - cgroup_kn_unlock(of->kn);
    - schedule();
    - finish_wait(&child->offline_waitq, &wait);
    - cgroup_put(child);
    -
    - return restart_syscall();
    - }
    - } while_each_subsys_mask();
    -
    cgrp->subtree_control = new_sc;
    cgrp->subtree_ss_mask = new_ss;

    --
    2.5.0
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