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    SubjectRe: [PATCH v2 1/6] perf/core: Fix incosistency between cgroup sched_out and sched_in
    On Tue, Mar 22, 2022 at 08:08:29PM +0800, Chengming Zhou wrote:
    > There is a race problem that can trigger WARN_ON_ONCE(cpuctx->cgrp)
    > in perf_cgroup_switch().
    >
    > CPU1 CPU2
    > (in context_switch) (attach running task)
    > perf_cgroup_sched_out(prev, next)
    > cgrp1 == cgrp2 is True
    > next->cgroups = cgrp3
    > perf_cgroup_attach()
    > perf_cgroup_sched_in(prev, next)
    > cgrp1 == cgrp3 is False
    >
    > The commit a8d757ef076f ("perf events: Fix slow and broken cgroup
    > context switch code") would save cpuctx switch out/in when the
    > perf_cgroup of "prev" and "next" are the same.
    >
    > But perf_cgroup of task can change in concurrent with context_switch.

    Can you clarify? IIRC then a task changes cgroup it goes throught the
    whole ->attach() dance, and that serializes against the context switch
    code.

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