在 2026/7/15 06:09, Tejun Heo 写道:
> Hello, Waiman.
>
>> + usleep(1000);
>> if (cg_check_frozen(cgroup, true))
>> goto cleanup;
>
> A fixed 1ms sleep only hides the race. On a loaded machine or a slow arch
> (you mention ppc64) the refreeze can take longer than 1ms, and the test
> reads the unfrozen state and fails anyway.
>
> The freezer test already has a way to wait for this properly.
> cg_prepare_for_wait() sets up an inotify watch on cgroup.events and
> cg_wait_for() blocks until it changes. cg_enter_and_wait_for_frozen() shows
> the pattern: loop cg_wait_for() then cg_check_frozen(). The cgroup always
> ends up frozen, so looping until cg_check_frozen() reports frozen is
> reliable and doesn't depend on timing.
>
> Can you respin using that instead of usleep()?
>
> Also, temporaily -> temporarily, in both the changelog and the comment.
>
Hi Tejun, Waiman,
I ran into a similar issue a while back and can add a data point on
the reproducibility: when running the full cgroup selftest suite, a
few cases -- test_cgfreezer_ptrace and test_cgfreezer_stopped -- fail
intermittently (around 40-70% on VMs), yet each failing case passes
reliably when run on its own. That points at state carried across
tests rather than a per-test bug, which is also why a fixed sleep/retry
is fragile.
On the "unfrozen state" above, I traced where CGRP_FROZEN actually gets
cleared. When the frozen tracee is woken by PTRACE_INTERRUPT,
JOBCTL_TRAP_STOP takes priority over JOBCTL_TRAP_FREEZE in get_signal(),
so the task enters ptrace_stop() instead of going back to
do_freezer_trap(). With debug printk in cgroup_update_frozen_flag() and
cgroup_leave_frozen(), the cg_test_ptrace trace shows:
0 -> 1 nr_frozen=1 task_count=1 /* initial freeze */
1 -> 0 nr_frozen=0 task_count=1 /* dec, task still in cgroup */
0 -> 1 nr_frozen=0 task_count=0 /* task left cgroup */
The 1 -> 0 step (nr_frozen 1->0 while task_count is still 1) is
cgroup_dec_frozen_cnt(), called from cgroup_leave_frozen(true) at the
end of ptrace_stop() (signal.c:2479) when the tracee is woken by
PTRACE_DETACH. That clears CGRP_FROZEN until the task loops back into
do_freezer_trap() and re-enters the frozen state -- the transient
unfrozen window the test hits.
As a kernel-side attempt I changed cgroup_enter_frozen() so it no longer
returns early when current->frozen is already true: css_set_lock is
taken before the check and, if CGRP_FREEZE is still set,
cgroup_update_frozen() is called to re-verify the cgroup frozen state
when a frozen task is handed off to ptrace.
Due to other work I've had to pause this investigation for now, so I'm
sharing the above as a data point rather than a finished fix.
Thanks,
Tao
> Thanks.
>
