linux-kernel - Re: [PATCH v2 2/2] prctl: propagate has_child

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Date:   Mon, 30 Jan 2017 16:15:54 +0300
From:   Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
To:     Oleg Nesterov <oleg@...hat.com>
CC:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Michal Hocko <mhocko@...e.com>,
        Stanislav Kinsburskiy <skinsbursky@...tuozzo.com>,
        "Mateusz Guzik" <mguzik@...hat.com>,
        <linux-kernel@...r.kernel.org>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Konstantin Khorenko <khorenko@...tuozzo.com>,
        "Lennart Poettering" <lennart@...ttering.net>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Kay Sievers <kay.sievers@...y.org>
Subject: Re: [PATCH v2 2/2] prctl: propagate has_child_subreaper flag to every
 descendant



On 01/30/2017 03:51 PM, Oleg Nesterov wrote:
> On 01/27, Pavel Tikhomirov wrote:
>>
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -1725,6 +1725,8 @@ struct task_struct {
>>  	struct signal_struct *signal;
>>  	struct sighand_struct *sighand;
>>
>> +	struct list_head csr_descendant;
>> +
>
> You forgot to remove this part ;)

Oh, sure.

>
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -1367,9 +1367,6 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
>>  	sig->oom_score_adj = current->signal->oom_score_adj;
>>  	sig->oom_score_adj_min = current->signal->oom_score_adj_min;
>>
>> -	sig->has_child_subreaper = current->signal->has_child_subreaper ||
>> -				   current->signal->is_child_subreaper;
>> -
>>  	mutex_init(&sig->cred_guard_mutex);
>>
>>  	return 0;
>> @@ -1800,6 +1797,13 @@ static __latent_entropy struct task_struct *copy_process(
>>
>>  			p->signal->leader_pid = pid;
>>  			p->signal->tty = tty_kref_get(current->signal->tty);
>> +			/*
>> +			 * Inherit has_child_subreaper flag under the same
>> +			 * tasklist_lock with adding child to the process tree
>> +			 * for propagate_has_child_subreaper optimization.
>> +			 */
>> +			p->signal->has_child_subreaper = current->signal->has_child_subreaper ||
>> +							 current->signal->is_child_subreaper;
>
> Ah yes, we need this change too...
>
> And perhaps it would be more correct to do
>
> 			p->signal->has_child_subreaper =
> 				p->real_parent->signal->has_child_subreaper ||
> 				p->real_parent->signal->is_child_subreaper;
>
> the current code is not exactly right if CLONE_PARENT...

I'm fine with inheriting 'has' flag from real_parent, because if 
real_parent does not have 'has' flag set but parent has 'has' set, we 
inherited the flag in vain.

But I don't actually think that inheritance from parent not real_parent 
breaks my optimization: if real_parent has the flag, so does the parent.

>
> Oleg.
>

-- 
Best regards, Tikhomirov Pavel
Software Developer, Virtuozzo.