linux-kernel - Re: [RFC,PATCH 2/2] cputimers/proc: do_task_stat()->thread_group

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Message-ID: <1269464082.12097.3.camel@laptop>
Date:	Wed, 24 Mar 2010 21:54:42 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Americo Wang <xiyou.wangcong@...il.com>,
	Balbir Singh <balbir@...ibm.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	Roland McGrath <roland@...hat.com>,
	Spencer Candland <spencer@...ehost.com>,
	Stanislaw Gruszka <sgruszka@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC,PATCH 2/2] cputimers/proc:
 do_task_stat()->thread_group_times() is racy and O(n) under ->siglock

On Wed, 2010-03-24 at 21:45 +0100, Oleg Nesterov wrote:
> Nowadays ->siglock is overloaded, it would be really nice to change
> do_task_stat() to walk through the list of threads lockless. And note
> that we are doing while_each_thread() twice!
> 
> while_each_thread() is rcu-safe, but thread_group_times() also needs
> ->siglock to serialize the modifications of signal_struct->prev_Xtime
> members.
> 
> 

Right, so from what I remember the issue is that, yes top et al rely on
that monotonicity, but more importantly I think
clock_gettime(CLOCK_PROCESS_CPUTIME_ID) should indeed use ->siglock to
ensure it serializes against do_exit() so that either we iterate the
thread or get the accumulated runtime from signal_struct but not both
(or neither).



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