linux-kernel - Re: [ckrm-tech] [PATCH 1/7] containers (V7): Generic container system abstracted from cpusets code

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Date:	Sat, 24 Mar 2007 21:45:50 -0700
From:	Paul Jackson <pj@....com>
To:	vatsa@...ibm.com
Cc:	sekharan@...ibm.com, ckrm-tech@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, xemul@...ru, dev@...ru,
	rohitseth@...gle.com, ebiederm@...ssion.com, mbligh@...gle.com,
	winget@...gle.com, containers@...ts.osdl.org, serue@...ibm.com,
	menage@...gle.com, devel@...nvz.org
Subject: Re: [ckrm-tech] [PATCH 1/7] containers (V7): Generic container
 system abstracted from cpusets code

vatsa wrote:
> Now consider:

Nice work - thanks.  Yes, both an extra cpuset count and a negative
cpuset count are bad news, opening the door to the usual catastrophes.

Would you like the honor of submitting the patch to add a task_lock
to cpuset_exit()?  If you do, be sure to fix, or at least remove,
the cpuset_exit comment lines:

 * We don't need to task_lock() this reference to tsk->cpuset,
 * because tsk is already marked PF_EXITING, so attach_task() won't
 * mess with it, or task is a failed fork, never visible to attach_task.

I guess that taking task_lock() in cpuset_exit() should not be a serious
performance issue.  It's taking a spinlock that is in the current
exiting tasks task struct, so it should be a cache hot memory line and
a rarely contested lock.

And I guess I've not see this race in real life, as one side of it has
to execute quite a bit of code in the task exit path, from when it sets
PF_EXITING until it gets into the cpuset_exit() call, while the other side
does the three lines:

	if (tsk->flags & PF_EXITING) ...
	atomic_inc(&cs->count);
	rcu_assign_pointer(tsk->cpuset, cs);

So, in real life, this would be a difficult race to trigger.

Thanks for finding this.

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@....com> 1.925.600.0401
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