linux-kernel - Re: [PATCH 2/2] oom: kill the insufficient and no longer needed PT_TRACE

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Message-ID: <20141202093530.GC27014@dhcp22.suse.cz>
Date:	Tue, 2 Dec 2014 10:35:30 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Cong Wang <xiyou.wangcong@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] oom: kill the insufficient and no longer needed
 PT_TRACE_EXIT check

On Fri 28-11-14 00:04:08, Oleg Nesterov wrote:
> After the previous patch we can remove the PT_TRACE_EXIT check in
> oom_scan_process_thread(), it was added to handle the case when the
> coredumping was "frozen" by ptrace, but it doesn't really work. If
> nothing else, we would need to check all threads which could share
> the same ->mm to make it more or less correct.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

I still do not see why we do not need task->ptrace & PT_TRACE_EXIT check
here. I do understand that the check on group_leader doesn't make much
sense. ptrace_event would block until the tracer let the task run again
which may be never AFAICS.

It is really sad how subtle and racy are all these checks :/

> ---
>  mm/oom_kill.c |   11 +++--------
>  1 files changed, 3 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 7af33b5..a2a4036 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -287,14 +287,9 @@ enum oom_scan_t oom_scan_process_thread(struct task_struct *task,
>  	if (oom_task_origin(task))
>  		return OOM_SCAN_SELECT;
>  
> -	if (task_will_free_mem(task) && !force_kill) {
> -		/*
> -		 * If this task is not being ptraced on exit, then wait for it
> -		 * to finish before killing some other task unnecessarily.
> -		 */
> -		if (!(task->group_leader->ptrace & PT_TRACE_EXIT))
> -			return OOM_SCAN_ABORT;
> -	}
> +	if (task_will_free_mem(task) && !force_kill)
> +		return OOM_SCAN_ABORT;
> +
>  	return OOM_SCAN_OK;
>  }
>  
> -- 
> 1.5.5.1
> 

-- 
Michal Hocko
SUSE Labs
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