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    SubjectRe: [PATCH 1/2] sched/cgroup: move sched_online_group() back into css_online()
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    On 26.01.2017 13:17, Peter Zijlstra wrote:
    > On Thu, Jan 26, 2017 at 12:41:41PM +0300, Konstantin Khlebnikov wrote:
    >> Commit 2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init") moved
    >> sched_online_group() from css_online() to css_alloc(). It exposes half-baked
    >> task group into global lists before initializing generic cgroup stuff.
    >>
    >> LTP testcase (third in cgroup_regression_test) written for testing
    >> similar race in kernels 2.6.26-2.6.28 easily triggers this oops:
    >>
    >
    > So nobody's run LTP against the kernel for almost a year?

    Yep. Nobody runs LTP =)

    CONFIG_RT_GROUP_SCHED must be y (which almost impossible to use IRL, I have some patches for it out of tree)

    Also systemd by default binds cgroups cpu and cpuacct together - this breaks testcase.

    >
    >>
    >> Here task group already linked into global RCU-protected list task_groups
    >> but pointer css->cgroup is still NULL.
    >>
    >> This patch reverts this chunk and moves online back to css_online().
    >
    > Maybe put a comment with it that explains why this is needed?
    >
    > Something along the lines of this perhaps?
    >

    Actually online is called before complete initialization. See second patch. =)
    I don't know what to do with this. cgroups are messy as always.

    > /*
    > * Don't expose the cgroup until initialization of it is complete in the
    > * cgroup core. Otherwise things like cgroup_path() will return NULL
    > * pointers and the like.
    > */
    >
    >> +static int cpu_cgroup_css_online(struct cgroup_subsys_state *css)
    >> +{
    >> + struct task_group *tg = css_tg(css);
    >> + struct task_group *parent = css_tg(css->parent);
    >> +
    >> + if (parent)
    >> + sched_online_group(tg, parent);
    >> + return 0;
    >> +}

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