linux-kernel - Re: [RFC] decrease tsk->signal->live before profile_task

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Date:   Tue, 9 Jun 2020 02:30:02 +0000
From:   "liuchao (CR)" <liuchao173@...wei.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
CC:     "mingo@...nel.org" <mingo@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Hushiyuan <hushiyuan@...wei.com>,
        "hewenliang (C)" <hewenliang4@...wei.com>
Subject: Re: [RFC] decrease tsk->signal->live before profile_task_exit

Eric W. Biederman <mailto:ebiederm@...ssion.com> writes:

> liuchao <liuchao173@...wei.com> writes:
> 
> > I want to dermine which thread is the last one to enter do_exit in
> > profile_task_exit. But when a lot of threads exit, tsk->signal->live
> > is not correct since it decrease after profile_task_exit.
> 
> I don't think that would be wise.
> 
> Any additional code before the sanity checks at the start of do_exit seems
> like a bad idea.
> 
> We could probably move the decrement of tsk->signal->live a little earlier,
> but not that much earlier in the function.
> 
> Does profile_task_exit even make sense that early in the code?  If the code
> is doing much of anything that is a completely inappopriate placement of
> profile_task_exit.

I think so too.

Move the decrement of tsk->signal->live after the sanity checks, then
profile_task_exit and kcov_task_exit make more sense.

> 
> Eric
> 
> 
> > Signed-off-by: liuchao <liuchao173@...wei.com>
> > ---
> >  kernel/exit.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/exit.c b/kernel/exit.c index
> > ce2a75bc0ade..1693764bc356 100644
> > --- a/kernel/exit.c
> > +++ b/kernel/exit.c
> > @@ -708,6 +708,7 @@ void __noreturn do_exit(long code)
> >  	struct task_struct *tsk = current;
> >  	int group_dead;
> >
> > +	group_dead = atomic_dec_and_test(&tsk->signal->live);
> >  	profile_task_exit(tsk);
> >  	kcov_task_exit(tsk);
> >
> > @@ -755,7 +756,6 @@ void __noreturn do_exit(long code)
> >  	if (tsk->mm)
> >  		sync_mm_rss(tsk->mm);
> >  	acct_update_integrals(tsk);
> > -	group_dead = atomic_dec_and_test(&tsk->signal->live);
> >  	if (group_dead) {
> >  		/*
> >  		 * If the last thread of global init has exited, panic