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

From: KAMEZAWA Hiroyuki
Date: Tue Aug 22 2006 - 18:20:53 EST

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

> KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> 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.


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