linux-kernel - Re: [PATCH v2] kernel/exit: do panic earlier to get coredump if global init task exit

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Date:   Tue, 17 Dec 2019 22:56:20 +0800
From:   chenqiwu <qiwuchen55@...il.com>
To:     Oleg Nesterov <oleg@...hat.com>, christian.brauner@...ntu.com,
        peterz@...radead.org, mingo@...nel.org
Cc:     kernel-team@...roid.com, linux-kernel@...r.kernel.org,
        chenqiwu@...omi.com
Subject: Re: [PATCH v2] kernel/exit: do panic earlier to get coredump if
 global init task exit

On Tue, Dec 17, 2019 at 03:25:15PM +0100, Oleg Nesterov wrote:
> On 12/17, chenqiwu wrote:
> >
> > @@ -728,6 +724,15 @@ void __noreturn do_exit(long code)
> >                 panic("Attempted to kill the idle task!");
> >
> >         /*
> > +        * If all threads of global init have exited, do panic imeddiately
> > +        * to get the coredump to find any clue for init task in userspace.
> > +        */
> > +       if (unlikely(is_global_init(tsk) &&
> > +               (atomic_read(&tsk->signal->live) == 1)))
> 
> Well, I guess this will work in practice, but in theory this is racy.
> 
> Suppose that signal->live == 2 and both threads exit in parallel. They
> both can see tsk->signal->live == 2 before atomic_dec_and_test().
> 
> If you are fine with this race I won't object, but please add a comment.
> 
> But why can't you simply do
> 
> 	--- x/kernel/exit.c
> 	+++ x/kernel/exit.c
> 	@@ -786,6 +786,8 @@ void __noreturn do_exit(long code)
> 		acct_update_integrals(tsk);
> 		group_dead = atomic_dec_and_test(&tsk->signal->live);
> 		if (group_dead) {
> 	+		if (unlikely(is_global_init(tsk)
> 	+			panic(...);
> 	 #ifdef CONFIG_POSIX_TIMERS
> 			hrtimer_cancel(&tsk->signal->real_timer);
> 			exit_itimers(tsk->signal);
> 
> ?
> 
> Oleg.
>

Oh, yeah, thanks for your reminds! But in fact, I think atomic_read()
can avoid the racy even if both threads exit in parallel, since it is
an atomic operation forever. I agree your simply modify, is there any
other questions?