Re: [PATCH v2 1/6] perf/core: Fix incosistency between cgroup sched_out and sched_in

[Peter Zijlstra]

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.