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    SubjectRe: [PATCH v5 4/7] cgroup: cgroup v2 freezer
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    On Tue, Dec 11, 2018 at 05:26:32PM +0100, Oleg Nesterov wrote:
    > On 12/07, Roman Gushchin wrote:
    > >
    > > Cgroup v2 freezer tries to put tasks into a state similar to jobctl
    > > stop. This means that tasks can be killed, ptraced (using
    > > PTRACE_SEIZE*), and interrupted. It is possible to attach to
    > > a frozen task, get some information (e.g. read registers) and detach.
    >
    > I fail to understand how this all supposed to work.
    >
    > > @@ -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) && p->jobctl == JOBCTL_TRAP_FREEZE))
    > > + return __fatal_signal_pending(p);
    >
    > I think I will never agree with this change ;) and I don't think it actually helps.

    See below.

    >
    > > +void cgroup_enter_frozen(void)
    > > +{
    > > + if (!current->frozen) {
    > > + spin_lock_irq(&css_set_lock);
    > > + current->frozen = true;
    > > + cgroup_inc_frozen_cnt(task_dfl_cgroup(current), false, true);
    > > + spin_unlock_irq(&css_set_lock);
    > > + }
    > > +
    > > + __set_current_state(TASK_INTERRUPTIBLE);
    > > + schedule();
    >
    > So once again, suppose it races with PTRACE_INTERRUPT, or SIGSTOP, or something
    > else which should be handled by get_signal() before do_freezer_trap().
    >
    > If (say) PTRACE_INTERRUPT comes before schedule it will be lost. Otherwise
    > the frozen task will react. This can't be right. Or I am totally confused.

    Why?
    PTRACE_INTERRUPT will set JOBCTL_TRAP_STOP, so signal_pending_state()
    will return true, schedule() will return immediately, and we'll handle the trap.

    >
    > Perhaps you can split this patch? start with cgroup_enter_frozen() using
    > TASK_KILLABLE, then teach it to handle ptrace/stop/etc? I think this way it
    > would be simpler to discuss the necessary changes and document what exactly
    > are you trying to do.
    >
    > and btw.... what about suspend? try_to_freeze_tasks() will obviously fail
    > if there is a ->frozen thread?

    I have to think a bit more here, but something like this will probably work:

    diff --git a/kernel/freezer.c b/kernel/freezer.c
    index b162b74611e4..590ac4d10b02 100644
    --- a/kernel/freezer.c
    +++ b/kernel/freezer.c
    @@ -134,7 +134,7 @@ bool freeze_task(struct task_struct *p)
    return false;

    spin_lock_irqsave(&freezer_lock, flags);
    - if (!freezing(p) || frozen(p)) {
    + if (!freezing(p) || frozen(p) || cgroup_task_frozen()) {
    spin_unlock_irqrestore(&freezer_lock, flags);
    return false;
    }
    --

    If the task is already frozen by the cgroup freezer, we don't have to do
    anything additionally.

    Thanks!

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