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    Subject[PATCH 4.14 072/109] cgroup: Fix dom_cgrp propagation when enabling threaded mode
    Date
    4.14-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Tejun Heo <tj@kernel.org>

    commit 479adb89a97b0a33e5a9d702119872cc82ca21aa upstream.

    A cgroup which is already a threaded domain may be converted into a
    threaded cgroup if the prerequisite conditions are met. When this
    happens, all threaded descendant should also have their ->dom_cgrp
    updated to the new threaded domain cgroup. Unfortunately, this
    propagation was missing leading to the following failure.

    # cd /sys/fs/cgroup/unified
    # cat cgroup.subtree_control # show that no controllers are enabled

    # mkdir -p mycgrp/a/b/c
    # echo threaded > mycgrp/a/b/cgroup.type

    At this point, the hierarchy looks as follows:

    mycgrp [d]
    a [dt]
    b [t]
    c [inv]

    Now let's make node "a" threaded (and thus "mycgrp" s made "domain threaded"):

    # echo threaded > mycgrp/a/cgroup.type

    By this point, we now have a hierarchy that looks as follows:

    mycgrp [dt]
    a [t]
    b [t]
    c [inv]

    But, when we try to convert the node "c" from "domain invalid" to
    "threaded", we get ENOTSUP on the write():

    # echo threaded > mycgrp/a/b/c/cgroup.type
    sh: echo: write error: Operation not supported

    This patch fixes the problem by

    * Moving the opencoded ->dom_cgrp save and restoration in
    cgroup_enable_threaded() into cgroup_{save|restore}_control() so
    that mulitple cgroups can be handled.

    * Updating all threaded descendants' ->dom_cgrp to point to the new
    dom_cgrp when enabling threaded mode.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    Reported-and-tested-by: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
    Reported-by: Amin Jamali <ajamali@pivotal.io>
    Reported-by: Joao De Almeida Pereira <jpereira@pivotal.io>
    Link: https://lore.kernel.org/r/CAKgNAkhHYCMn74TCNiMJ=ccLd7DcmXSbvw3CbZ1YREeG7iJM5g@mail.gmail.com
    Fixes: 454000adaa2a ("cgroup: introduce cgroup->dom_cgrp and threaded css_set handling")
    Cc: stable@vger.kernel.org # v4.14+
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    include/linux/cgroup-defs.h | 1 +
    kernel/cgroup/cgroup.c | 25 ++++++++++++++++---------
    2 files changed, 17 insertions(+), 9 deletions(-)

    --- a/include/linux/cgroup-defs.h
    +++ b/include/linux/cgroup-defs.h
    @@ -353,6 +353,7 @@ struct cgroup {
    * specific task are charged to the dom_cgrp.
    */
    struct cgroup *dom_cgrp;
    + struct cgroup *old_dom_cgrp; /* used while enabling threaded */

    /*
    * list of pidlists, up to two for each namespace (one for procs, one
    --- a/kernel/cgroup/cgroup.c
    +++ b/kernel/cgroup/cgroup.c
    @@ -2780,11 +2780,12 @@ restart:
    }

    /**
    - * cgroup_save_control - save control masks of a subtree
    + * cgroup_save_control - save control masks and dom_cgrp of a subtree
    * @cgrp: root of the target subtree
    *
    - * Save ->subtree_control and ->subtree_ss_mask to the respective old_
    - * prefixed fields for @cgrp's subtree including @cgrp itself.
    + * Save ->subtree_control, ->subtree_ss_mask and ->dom_cgrp to the
    + * respective old_ prefixed fields for @cgrp's subtree including @cgrp
    + * itself.
    */
    static void cgroup_save_control(struct cgroup *cgrp)
    {
    @@ -2794,6 +2795,7 @@ static void cgroup_save_control(struct c
    cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
    dsct->old_subtree_control = dsct->subtree_control;
    dsct->old_subtree_ss_mask = dsct->subtree_ss_mask;
    + dsct->old_dom_cgrp = dsct->dom_cgrp;
    }
    }

    @@ -2819,11 +2821,12 @@ static void cgroup_propagate_control(str
    }

    /**
    - * cgroup_restore_control - restore control masks of a subtree
    + * cgroup_restore_control - restore control masks and dom_cgrp of a subtree
    * @cgrp: root of the target subtree
    *
    - * Restore ->subtree_control and ->subtree_ss_mask from the respective old_
    - * prefixed fields for @cgrp's subtree including @cgrp itself.
    + * Restore ->subtree_control, ->subtree_ss_mask and ->dom_cgrp from the
    + * respective old_ prefixed fields for @cgrp's subtree including @cgrp
    + * itself.
    */
    static void cgroup_restore_control(struct cgroup *cgrp)
    {
    @@ -2833,6 +2836,7 @@ static void cgroup_restore_control(struc
    cgroup_for_each_live_descendant_post(dsct, d_css, cgrp) {
    dsct->subtree_control = dsct->old_subtree_control;
    dsct->subtree_ss_mask = dsct->old_subtree_ss_mask;
    + dsct->dom_cgrp = dsct->old_dom_cgrp;
    }
    }

    @@ -3140,6 +3144,8 @@ static int cgroup_enable_threaded(struct
    {
    struct cgroup *parent = cgroup_parent(cgrp);
    struct cgroup *dom_cgrp = parent->dom_cgrp;
    + struct cgroup *dsct;
    + struct cgroup_subsys_state *d_css;
    int ret;

    lockdep_assert_held(&cgroup_mutex);
    @@ -3169,12 +3175,13 @@ static int cgroup_enable_threaded(struct
    */
    cgroup_save_control(cgrp);

    - cgrp->dom_cgrp = dom_cgrp;
    + cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp)
    + if (dsct == cgrp || cgroup_is_threaded(dsct))
    + dsct->dom_cgrp = dom_cgrp;
    +
    ret = cgroup_apply_control(cgrp);
    if (!ret)
    parent->nr_threaded_children++;
    - else
    - cgrp->dom_cgrp = cgrp;

    cgroup_finalize_control(cgrp, ret);
    return ret;

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