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

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Date:   Fri, 29 May 2020 13:01:22 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     liuchao <liuchao173@...wei.com>
Cc:     <mingo@...nel.org>, <tglx@...utronix.de>,
        <linux-kernel@...r.kernel.org>, <hushiyuan@...wei.com>,
        <hewenliang4@...wei.com>
Subject: Re: [RFC] decrease tsk->signal->live before profile_task_exit

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.

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