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    Subject[PATCH v9 1/7] cgroup/cpuset: Don't let child cpusets restrict parent in default hierarchy
    Date
    In validate_change(), there is a check since v2.6.12 to make sure that
    each of the child cpusets must be a subset of a parent cpuset. IOW, it
    allows child cpusets to restrict what changes can be made to a parent's
    "cpuset.cpus". This actually violates one of the core principles of the
    default hierarchy where a cgroup higher up in the hierarchy should be
    able to change configuration however it sees fit as deligation breaks
    down otherwise.

    To address this issue, the check is now removed for the default hierarchy
    to free parent cpusets from being restricted by child cpusets. The
    check will still apply for legacy hierarchy.

    Suggested-by: Tejun Heo <tj@kernel.org>
    Signed-off-by: Waiman Long <longman@redhat.com>
    ---
    kernel/cgroup/cpuset.c | 14 +++-----------
    1 file changed, 3 insertions(+), 11 deletions(-)

    diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
    index d0e163a02099..0dd7d853ed17 100644
    --- a/kernel/cgroup/cpuset.c
    +++ b/kernel/cgroup/cpuset.c
    @@ -616,19 +616,11 @@ static int validate_change(struct cpuset *cur, struct cpuset *trial)
    struct cpuset *c, *par;
    int ret;

    - rcu_read_lock();
    -
    - /* Each of our child cpusets must be a subset of us */
    - ret = -EBUSY;
    - cpuset_for_each_child(c, css, cur)
    - if (!is_cpuset_subset(c, trial))
    - goto out;
    -
    - /* Remaining checks don't apply to root cpuset */
    - ret = 0;
    + /* The checks don't apply to root cpuset */
    if (cur == &top_cpuset)
    - goto out;
    + return 0;

    + rcu_read_lock();
    par = parent_cs(cur);

    /* On legacy hierarchy, we must be a subset of our parent cpuset. */
    --
    2.27.0
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