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SubjectRe: [PATCH v4 4/7] cgroup: cgroup v2 freezer
To be honest, I fail to understand this patch. At least after a quick glance,
I will try to read it again tomorrow but so far I do not even understand the
desired semantics wrt signals/ptrace.

On 11/30, Roman Gushchin wrote:
>
> @@ -368,6 +369,8 @@ static inline int signal_pending_state(long state, struct task_struct *p)
> return 0;
> if (!signal_pending(p))
> return 0;
> + if (unlikely(cgroup_task_frozen(p)))
> + return __fatal_signal_pending(p);

Oh, this is not nice. And doesn't look right.

> +/*
> + * Entry path into frozen state.
> + * If the task was not frozen before, counters are updated and the cgroup state
> + * is revisited. Otherwise, the task is put into the TASK_KILLABLE sleep.
> + */
> +void cgroup_enter_frozen(void)
> +{
> + if (!current->frozen) {
> + struct cgroup *cgrp;
> +
> + spin_lock_irq(&css_set_lock);
> + current->frozen = true;
> + cgrp = task_dfl_cgroup(current);
> + cgrp->freezer.nr_frozen_tasks++;
> + WARN_ON_ONCE(cgrp->freezer.nr_frozen_tasks >
> + cgrp->freezer.nr_tasks_to_freeze);
> + cgroup_update_frozen(cgrp, true);
> + spin_unlock_irq(&css_set_lock);
> + }
> +
> + __set_current_state(TASK_INTERRUPTIBLE);
> + schedule();

The comment above says TASK_KILLABLE, very confusing.

Probably this pairs with the change in signal_pending_state() above. So this
schedule() should actually "work" in that it won't return if signal_pending().

But this can't protect from another signal_wake_up(). Yes, iiuc in this case
cgroup_enter_frozen() will be called again "soon" but this all looks strange.

> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -410,6 +410,13 @@ static int ptrace_attach(struct task_struct *task, long request,
>
> spin_lock(&task->sighand->siglock);
>
> + /*
> + * If the process is frozen, let's wake it up to give it a chance
> + * to enter the ptrace trap.
> + */
> + if (cgroup_task_frozen(task))
> + wake_up_process(task);

And why this can't race with cgroup_enter_frozen() ?

Or think of PTRACE_INTERRUPT. It can race with cgroup_enter_frozen() too, the
tracee can miss this request because of that change in signal_pending_state().


> static void do_jobctl_trap(void)
> {
> + struct sighand_struct *sighand = current->sighand;
> struct signal_struct *signal = current->signal;
> int signr = current->jobctl & JOBCTL_STOP_SIGMASK;
>
> - if (current->ptrace & PT_SEIZED) {
> - if (!signal->group_stop_count &&
> - !(signal->flags & SIGNAL_STOP_STOPPED))
> - signr = SIGTRAP;
> - WARN_ON_ONCE(!signr);
> - ptrace_do_notify(signr, signr | (PTRACE_EVENT_STOP << 8),
> - CLD_STOPPED);
> - } else {
> - WARN_ON_ONCE(!signr);
> - ptrace_stop(signr, CLD_STOPPED, 0, NULL);
> - current->exit_code = 0;
> + if (current->jobctl & (JOBCTL_TRAP_STOP | JOBCTL_TRAP_NOTIFY)) {
> + if (current->ptrace & PT_SEIZED) {
> + if (!signal->group_stop_count &&
> + !(signal->flags & SIGNAL_STOP_STOPPED))
> + signr = SIGTRAP;
> + WARN_ON_ONCE(!signr);
> + ptrace_do_notify(signr,
> + signr | (PTRACE_EVENT_STOP << 8),
> + CLD_STOPPED);
> + } else {
> + WARN_ON_ONCE(!signr);
> + ptrace_stop(signr, CLD_STOPPED, 0, NULL);
> + current->exit_code = 0;
> + }
> + } else if (current->jobctl & JOBCTL_TRAP_FREEZE) {
> + /*
> + * Enter the frozen state, unless the task is about to exit.
> + */
> + if (fatal_signal_pending(current)) {
> + current->jobctl &= ~JOBCTL_TRAP_FREEZE;
> + } else {
> + spin_unlock_irq(&sighand->siglock);
> + cgroup_enter_frozen();
> + spin_lock_irq(&sighand->siglock);
> + }
> }
> }
>
> @@ -2401,12 +2420,23 @@ bool get_signal(struct ksignal *ksig)
> do_signal_stop(0))
> goto relock;
>
> - if (unlikely(current->jobctl & JOBCTL_TRAP_MASK)) {
> + if (unlikely(current->jobctl &
> + (JOBCTL_TRAP_MASK | JOBCTL_TRAP_FREEZE))) {
> do_jobctl_trap();

Cosmetic nit, but can't you add another helper? To me something like

if (JOBCTL_TRAP_MASK)
do_jobctl_trap();
else if (JOBCTL_TRAP_FREEZE)
do_jobctl_freeze();

will look more clean, but I won't insist.


> + /*
> + * If the task is leaving the frozen state, let's update
> + * cgroup counters and reset the frozen bit.
> + */
> + if (unlikely(cgroup_task_frozen(current))) {
> + spin_unlock_irq(&sighand->siglock);
> + cgroup_leave_frozen();
> + spin_lock_irq(&sighand->siglock);

looks like this needs another "goto relock", no?

And perhaps this another reason for the new do_jobctl_freeze() helper which
could absorb this leave_frozen() ?

Oleg.

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