Re: [PATCH v2] selftests/cgroup: Fix intermittent test_cgfreezer_ptrace test failures

[Michal Koutný]

Hi.

On Tue, Jul 14, 2026 at 11:54:46PM -0400, Waiman Long <longman@xxxxxxxxxx> wrote:
> It is found that the test_cgfreezer_ptrace test of test_freezer can
> intermittently fail on some architectures like arm64 and ppc64.
> 
> After further tracing of the mechanics of the test_cgfreezer_ptrace test,
> it is found that the ptrace(PTRACE_DETACH) call will spawn another process
> to perform the detach 

I'm reading kernel/ptrace.c and cannot find a new task creation.
I'm curious what is this "another process"?

(Or is it referring to the child_fn() process from
test_cgfreezer_ptrace()? Then I'm suspicous about the transitional
unfreezing of it after PTRACE_DETACH. Wasn't it rightful expectation of
the test that the cgroup remains contiguously frozen?)

> by temporarily unfreezes the cgroup and then freezes
> it again afterward during the detaching process. The reading of the
> frozen flag from cgroup.events is done by the main test_freezer process
> running probably on a different CPU. As a result, racing is possible and
> the intermediate unfrozen state can be read leading to occasional test
> failures especially on architectures with a weak memory model like arm64.

What state would the task be in during this window?

Thanks,
Michal

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