Virtualization - May 2023 - [RFC PATCH 1/8] signal: Dequeue SIGKILL even if SIGNAL_GROUP_EXIT/group_exec_task is set

On 5/18/23 11:25 AM, Oleg Nesterov wrote:
> I too do not understand the 1st change in this patch ...
> 
> On 05/18, Mike Christie wrote:
>>
>> In the other patches we do:
>>
>> if (get_signal(ksig))
>> 	start_exit_cleanup_by_stopping_newIO()
>> 	flush running IO()
>> 	exit()
>>
>> But to do the flush running IO() part of this I need to wait for it so
>> that's why I wanted to be able to dequeue the SIGKILL and clear the
>> TIF_SIGPENDING bit.
> 
> But get_signal() will do what you need, dequeue SIGKILL and clear
SIGPENDING ?
> 
> 	if ((signal->flags & SIGNAL_GROUP_EXIT) ||
> 	     signal->group_exec_task) {
> 		clear_siginfo(&ksig->info);
> 		ksig->info.si_signo = signr = SIGKILL;
> 		sigdelset(&current->pending.signal, SIGKILL);
> 
> this "dequeues" SIGKILL,
> 
> 		trace_signal_deliver(SIGKILL, SEND_SIG_NOINFO,
> 			&sighand->action[SIGKILL - 1]);
> 		recalc_sigpending();
> 
> this clears TIF_SIGPENDING.
> 
I see what you guys meant. TIF_SIGPENDING isn't getting cleared.
I'll dig into why.

On 05/18, Mike Christie wrote:
>
> On 5/18/23 11:25 AM, Oleg Nesterov wrote:
> > I too do not understand the 1st change in this patch ...
> >
> > On 05/18, Mike Christie wrote:
> >>
> >> In the other patches we do:
> >>
> >> if (get_signal(ksig))
> >> 	start_exit_cleanup_by_stopping_newIO()
> >> 	flush running IO()
> >> 	exit()
> >>
> >> But to do the flush running IO() part of this I need to wait for
it so
> >> that's why I wanted to be able to dequeue the SIGKILL and
clear the
> >> TIF_SIGPENDING bit.
> >
> > But get_signal() will do what you need, dequeue SIGKILL and clear
SIGPENDING ?
> >
> > 	if ((signal->flags & SIGNAL_GROUP_EXIT) ||
> > 	     signal->group_exec_task) {
> > 		clear_siginfo(&ksig->info);
> > 		ksig->info.si_signo = signr = SIGKILL;
> > 		sigdelset(&current->pending.signal, SIGKILL);
> >
> > this "dequeues" SIGKILL,
OOPS. this doesn't remove SIGKILL from current->signal->shared_pending
> >
> > 		trace_signal_deliver(SIGKILL, SEND_SIG_NOINFO,
> > 			&sighand->action[SIGKILL - 1]);
> > 		recalc_sigpending();
> >
> > this clears TIF_SIGPENDING.
No, I was wrong, recalc_sigpending() won't clear TIF_SIGPENDING if
SIGKILL is in signal->shared_pending
> I see what you guys meant. TIF_SIGPENDING isn't getting cleared.
> I'll dig into why.
See above, sorry for confusion.



And again, there is another problem with SIGSTOP. To simplify, suppose
a PF_IO_WORKER thread does something like

	while (signal_pending(current))
		get_signal(...);

this will loop forever if (SIGNAL_GROUP_EXIT || group_exec_task) and
SIGSTOP is pending.

Oleg.