linux-kernel - Re: [PATCH -mm 2/4] cputimers: make sure thread_group_cputime() can't count the same thread twice lockless

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Date:	Tue, 30 Mar 2010 13:01:24 +0200
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Americo Wang <xiyou.wangcong@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Roland McGrath <roland@...hat.com>,
	Spencer Candland <spencer@...ehost.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm 2/4] cputimers: make sure thread_group_cputime()
	can't count the same thread twice lockless

On Mon, Mar 29, 2010 at 08:13:29PM +0200, Oleg Nesterov wrote:
> - change __exit_signal() to do __unhash_process() before we accumulate
>   the counters in ->signal
> 
> - add a couple of barriers into thread_group_cputime() and __exit_signal()
>   to make sure thread_group_cputime() can never account the same thread
>   twice if it races with exit.
>
>   If any thread T was already accounted in ->signal, next_thread() or
>   pid_alive() must see the result of __unhash_process(T).

Memory barriers prevents to account times twice, but as we write
sig->{u,s}time and sig->sum_sched_runtime on one cpu and read them
on another cpu, without a lock, this patch make theoretically possible
that some accumulated values of struct task_cputime may include exited
task values and some not. In example times->utime will include values
from 10 threads, and times->{stime,sum_exec_runtime} values form 9
threads, because local cpu updates sig->utime but not two other values.
This can make scaling in thread_group_times() not correct. I'm not sure
how big drawback is that.

Stanislaw
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