Re: [PATCH] sched: __fatal_signal_pending() should also check PF_EXITING

[Tycho Andersen <tycho@tycho.pizza>]

On Thu, Jul 28, 2022 at 11:12:20AM +0200, Oleg Nesterov wrote:
> This is clear, but it seems you do not understand me. Let me try again
> to explain and please correct me if I am wrong.
> 
> To simplify, lets suppose we have a single-thread task T which simply
> does
> 	__set_current_state(TASK_KILLABLE);
> 	schedule();
> 
> in the do_exit() paths after exit_signals() which sets PF_EXITING. Btw,
> note that it even documents that this thread is not "visible" for the
> group-wide signals, see below.
> 
> Now, suppose that this task is running and you send SIGKILL. T will
> dequeue SIGKILL from T->penging and call do_exit(). However, it won't
> remove SIGKILL from T->signal.shared_pending(), and this means that
> signal_pending(T) is still true.
> 
> Now. If we add a PF_EXITING or sigismember(shared_pending, SIGKILL) check
> into __fatal_signal_pending(), then yes, T won't block in schedule(),
> schedule()->signal_pending_state() will return true.
> 
> But what if T exits on its own? It will block in schedule() forever.
> schedule()->signal_pending_state() will not even check __fatal_signal_pending(),
> signal_pending() == F.
> 
> Now if you send SIGKILL to this task, SIGKILL won't wake it up or even
> set TIF_SIGPENDING, complete_signal() will do nothing.
> 
> See?
> 
> I agree, we should probably cleanup this logic and define how exactly
> the exiting task should react to signals (not only fatal signals). But
> your patch certainly doesn't look good to me and it is not enough.
> May be we can change get_signal() to not remove SIGKILL from t->pending
> for the start... not sure, this needs another discussion.

Thank you for this! Between that and Eric's line about:

> Frankly that there are some left over SIGKILL bits in the pending mask
> is a misfeature, and it is definitely not something you should count on.

I think I finally maybe understand the objections.

Is it fair to say that a task with PF_EXITING should never wait? I'm
wondering if a solution would be to patch the wait code to look for
PF_EXITING, in addition to checking the signal state.

> Finally. if fuse_flush() wants __fatal_signal_pending() == T when the
> caller exits, perhaps it can do it itself? Something like
> 
> 	if (current->flags & PF_EXITING) {
> 		spin_lock_irq(siglock);
> 		set_thread_flag(TIF_SIGPENDING);
> 		sigaddset(&current->pending.signal, SIGKILL);
> 		spin_unlock_irq(siglock);
> 	}
> 
> Sure, this is ugly as hell. But perhaps this can serve as a workaround?

or even just

    if (current->flags & PF_EXITING)
        return;

since we don't have anyone to send the result of the flush to anyway.
If we don't end up converging on a fix here, I'll just send that
patch. Thanks for the suggestion.

Tycho