Sorry, this version raced with my vacation, I missed it.
I'll try to read this code carefully but after a quick glance I have some
concerns,
On 12/21, Roman Gushchin wrote:
>
> +static void cgroup_update_frozen(struct cgroup *cgrp)
> +{
> + bool frozen;
> +
> + lockdep_assert_held(&css_set_lock);
> +
> + /*
> + * If the cgroup has to be frozen (CGRP_FREEZE bit set),
> + * and all tasks are frozen or stopped, let's consider
> + * the cgroup frozen. Otherwise it's not frozen.
> + */
> + frozen = test_bit(CGRP_FREEZE, &cgrp->flags) &&
> + cgrp->freezer.nr_frozen_tasks +
> + cgrp->freezer.nr_stopped_tasks ==
> + cgrp->freezer.nr_tasks_to_freeze;
OK. Suppose that cgroup is frozen, CGRP_FROZEN is set, stopped == 0,
to_freeze = frozen.
One of the task is killed, it calls leave_frozen(). If I read this code path
correctly, only ->nr_frozen_tasks will be decremented, so "frozen" will be
"false" when cgroup_update_frozen() is called.
Doesn't this mean that this cgroup will no longer be CGRP_FROZEN even after
the killed task goes away completely?
Or. Suppose that another process picks a task from the CGRP_FROZEN cgroup and
does PTRACE_ATTACH + PTRACE_INTERRUPT. IIUC, the tracee will only increment
->nr_stopped_tasks, it won't touch other counters. Again, cgroup won't be FROZEN
until PTRACE_CONT'ed tracee does cgroup_leave_stopped() ? This looks strange at
least.
SIGSTOP. IIUC, a frozen task sleeping in do_freezer_trap() won't stop. However if
another thread has already called do_signal_stop(), the woken frozen task will
react to JOBCTL_STOP_PENDING and stop. And do_signal_stop()->cgroup_enter_stopped()
will "destroy" CGRP_FROZEN too, or I am totally confused.
OTOH, if you freeze a TASK_STOPPED task's cgroup, this task can react to SIGCONT,
notify its parent, then freeze again. This is fine, but iiuc this cgroup won't be
FROZEN in between, cgroup_file_notify() will be called twice...
Oleg.
