linux-kernel - Re: [RFC][PATCH] ps command race fix take2 [1/4] list token

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Message-Id: <20060823072256.7d931f8b.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 23 Aug 2006 07:22:56 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	ebiederm@...ssion.com (Eric W. Biederman)
Cc:	linux-kernel@...r.kernel.org, akpm@...l.org, pj@....com,
	saito.tadashi@...t.fujitsu.com, ak@...e.de
Subject: Re: [RFC][PATCH] ps command race fix take2 [1/4] list token

On Tue, 22 Aug 2006 10:56:08 -0600
ebiederm@...ssion.com (Eric W. Biederman) wrote:

> KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> writes:
> 
> > This is ps command race fix take2. Unfortunately, against 2.6.18-rc4.
> > I'll rebase this to appropriate kernel if O.K. (I think this is RFC)
> >
> > This patch implements Paul Jackson's idea, 'inserting false link in task list'.
> 
> Currently the tasklist_lock is one of the more highly contended locks in
> the kernel.  Adding an extra place it is taken is undesirable.
yes. taking lock is a probem.
I know current readdir() uses 8192 bytes buffer for getdents64(). Then,
maybe write-lock will be acquired all-tgids/400+ times for inserting token
(in 32bit system).
 
> If could see a better algorithm for sending a signal to all processes
> in a process groups we could remove the tasklist_lock entirely.
> 
??
Sorry, could you explain more ?

> In addition you only solves half the readdir problems.  You don't solve
> the seek problem which is returning to an offset you had been to
> before.  A relatively rare case but...
> 
Ah, I should add lseek handler for proc root. Okay.

> > Good point of this approach is cost of searching task is O(N) (N=num of tgids).
> > Bad point is lock and kmalloc/kfree.
> > I didin't modified thread_list and cpuset's proc list, maybe future work.
> >
> > If searching pid bitmap is better, please take Erics.
> 
> My patch at least needs a good changelog but I believe it will work
> better and can be further improved with a better pid data structure
> if there is actually a problem there.  Given that I don't take
> any locks it should be much friendlier at scale, and the code
> was simpler.
yes. it has several good points and simple.
My patch's point is just using task_list if we can, because it exists for keeping
all tasks(tgids).

> 
> However I will miss a few newly forked processes and I don't think your
> technique will miss any.  Still neither will miss a process that
> existed the entire time.
> 
> If nothing else I think it was worth posting so we could contrast the two.
> 
please post again. I think comparing the two is good.
I will post take3 with improved comments and lseek handler, and so on.

-Kame 

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