Messages in this thread | | | Date | Wed, 7 Jul 2021 16:14:12 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH v2 4/4] freezer,sched: Rewrite core freezer logic |
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sorry for delay...
I am still trying to understand this series, just one note for now.
On 06/24, Peter Zijlstra wrote: > > +static bool __freeze_task(struct task_struct *p) > +{ > + unsigned long flags; > + unsigned int state; > + bool frozen = false; > + > + raw_spin_lock_irqsave(&p->pi_lock, flags); > + state = READ_ONCE(p->__state); > + if (state & (TASK_FREEZABLE|__TASK_STOPPED|__TASK_TRACED)) { > + /* > + * Only TASK_NORMAL can be augmented with TASK_FREEZABLE, > + * since they can suffer spurious wakeups. > + */ > + if (state & TASK_FREEZABLE) > + WARN_ON_ONCE(!(state & TASK_NORMAL)); > + > +#ifdef CONFIG_LOCKDEP > + /* > + * It's dangerous to freeze with locks held; there be dragons there. > + */ > + if (!(state & __TASK_FREEZABLE_UNSAFE)) > + WARN_ON_ONCE(debug_locks && p->lockdep_depth); > +#endif > + > + if (state & (__TASK_STOPPED|__TASK_TRACED)) > + WRITE_ONCE(p->__state, TASK_FROZEN|__TASK_FROZEN_SPECIAL);
Well, this doesn't look right.
Firstly, this can race with ptrace_freeze_traced() which can set p->__state = __TASK_TRACED and clear TASK_FROZEN. Or with __set_current_state(TASK_RUNNING) in ptrace_stop().
But the main problem is that you can't simply remove __TASK_TRACED, this can confuse the debugger, any ptrace() request will fail as if the tracee was killed.
Another problem. Suppose that p->parent sleeps in do_wait(). p calls ptrace_stop(), sets __TASK_TRACED, and wakes the parent up.
__freeze_task() clears __TASK_TRACED.
The parent calls wait_task_stopped(p) but it fails because task_is_traced() returns false. The parent sleeps again, and forever because __thaw_special() won't notify it.
Or. Suppose that __freeze_task() removes __TASK_STOPPED. The new debugger comes, the tracee should switch from STOPPED to TRACED. But this won't happen because task_is_stopped() in ptrace_() will return false and task_set_jobctl_pending/signal_wake_up_state won't be called.
Oleg.
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