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

Paul Jackson
2007-03-25T04:45:55Z

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@sgi.com> 1.925.600.0401