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SubjectRe: [PATCH v10 4/9] cgroup: cgroup v2 freezer
On 04/05, Roman Gushchin wrote:
>
> +void cgroup_leave_frozen(bool always_leave)
> +{
> + struct cgroup *cgrp;
> +
> + spin_lock_irq(&css_set_lock);
> + cgrp = task_dfl_cgroup(current);
> + if (always_leave || !test_bit(CGRP_FREEZE, &cgrp->flags)) {
> + cgroup_dec_frozen_cnt(cgrp);
> + cgroup_update_frozen(cgrp);
> + WARN_ON_ONCE(!current->frozen);
> + current->frozen = false;
> + }
> + spin_unlock_irq(&css_set_lock);
> +
> + if (unlikely(current->frozen)) {
> + /*
> + * If the task remained in the frozen state,
> + * make sure it won't reach userspace without
> + * entering the signal handling loop.
> + */
> + spin_lock_irq(&current->sighand->siglock);
> + recalc_sigpending();
> + spin_unlock_irq(&current->sighand->siglock);

I still can't understand this logic.

Once again, suppose we race with CGRP_FREEZE. If JOBCTL_TRAP_FREEZE is already
set then signal_pending() must be already T and we do not need recalc_sigpending?
If JOBCTL_TRAP_FREEZE is not set yet, how can recalc_sigpending() help?

> +static void cgroup_freeze_task(struct task_struct *task, bool freeze)
> +{
> + unsigned long flags;
> +
> + /* If the task is about to die, don't bother with freezing it. */
> + if (!lock_task_sighand(task, &flags))
> + return;
> +
> + if (freeze) {
> + task->jobctl |= JOBCTL_TRAP_FREEZE;
> + signal_wake_up(task, false);
> + } else {
> + task->jobctl &= ~JOBCTL_TRAP_FREEZE;
> + wake_up_process(task);

wake_up_interruptible() ?

> static int ptrace_signal(int signr, kernel_siginfo_t *info)
> {
> /*
> @@ -2442,6 +2483,10 @@ bool get_signal(struct ksignal *ksig)
> ksig->info.si_signo = signr = SIGKILL;
> sigdelset(&current->pending.signal, SIGKILL);
> recalc_sigpending();
> + current->jobctl &= ~JOBCTL_TRAP_FREEZE;
> + spin_unlock_irq(&sighand->siglock);
> + if (unlikely(cgroup_task_frozen(current)))
> + cgroup_leave_frozen(true);

Oh, and another leave_frozen below...

I feel this must be simplified somehow, but nothing comes to my mind right now.

> + /*
> + * 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(true);
> goto relock;
> }

afaics cgroup_leave_frozen(false) makes more sense here.

Oleg.

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