linux-kernel - Re: [PATCH 0/9] Containers (V9): Generic Process Containers

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Date:	Mon, 30 Apr 2007 23:36:53 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	"Paul Menage" <menage@...gle.com>
Cc:	"Paul Jackson" <pj@....com>, akpm@...ux-foundation.org, dev@...ru,
	xemul@...ru, serue@...ibm.com, ebiederm@...ssion.com,
	haveblue@...ibm.com, svaidy@...ux.vnet.ibm.com, balbir@...ibm.com,
	ckrm-tech@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	rohitseth@...gle.com, mbligh@...gle.com, containers@...ts.osdl.org,
	devel@...nvz.org
Subject: Re: [PATCH 0/9] Containers (V9): Generic Process Containers

On Mon, Apr 30, 2007 at 10:09:38AM -0700, Paul Menage wrote:
> Paul, is there any reason why we need to do a write_lock() on
> tasklist_lock if we're just trying to block fork, or is it just
> historical accident? Wouldn't it be fine to do a read_lock()?

Good point ..read_lock() will probably suffice in update_nodemask which
means we don't need the patch I sent earlier.

Paul (Jackson),
	This made me see another race in update_nodemask vs fork:

Lets say cpuset CS1 has only one task T1 to begin with.

update_nodemask(CS1)			T1 in do_fork()
	CPU0				     CPU1
=============================================================


					cpuset_fork();
					mpol_copy();


ntasks = atomic_read(&cs->count);
[ntasks = 2, accounting new born child T2]
cs->mems_allowed = something;
set_cpuset_being_rebound()


write/read_lock(tasklist_lock);


do_each_thread {

	/* Finds only T1 */

	mmarray[] = ..

} while_each_thread();

write/read_unlock(tasklist_lock);

					write_lock(tasklist_lock);

					/* Add T2, child of T1 to tasklist */

					write_unlock(tasklist_lock);


for (i = 0; i < n; i++) {

	mpol_rebind_mm(..);

}


In this for loop, we migrate only T1's ->mm. T2's->mm isn't migrated
AFAICS.

Is that fine?

-- 
Regards,
vatsa
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