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SubjectRe: [patch for-3.2-rc3] cpusets: stall when updating mems_allowed for mempolicy or disjoint nodemask
On Fri, 18 Nov 2011, Miao Xie wrote:

> >> I find these is another problem, please take account of the following case:
> >>
> >> 2-3 -> 1-2 -> 0-1
> >>
> >> the user change mems_allowed twice continuously, the task may see the empty
> >> mems_allowed.
> >>
> >> So, it is still dangerous.
> >>
> >
> > With this patch, we're protected by task_lock(tsk) to determine whether we
> > want to take the exception, i.e. whether need_loop is false, and the
> > setting of tsk->mems_allowed. So this would see the nodemask change at
> > the individual steps from 2-3 -> 1-2 -> 0-1, not some inconsistent state
> > in between or directly from 2-3 -> 0-1. The only time we don't hold
> > task_lock(tsk) to change tsk->mems_allowed is when tsk == current and in
> > that case we're not concerned about intermediate reads to its own nodemask
> > while storing to a mask where MAX_NUMNODES > BITS_PER_LONG.
> >
> > Thus, there's no problem here with regard to such behavior if we exclude
> > mempolicies, which this patch does.
> >
>
> No.
> When the task does memory allocation, it access its mems_allowed without
> task_lock(tsk), and it may be blocked after it check 0-1 bits. And then, the
> user changes mems_allowed twice continuously(2-3(initial state) -> 1-2 -> 0-1),
> After that, the task is woke up and it see the empty mems_allowed.
>

I'm confused, you're concerned on a kernel where
MAX_NUMNODES > BITS_PER_LONG about thread A reading a partial
tsk->mems_allowed, being preempted, meanwhile thread B changes
tsk->mems_allowed by taking cgroup_mutex, taking task_lock(tsk), setting
the intersecting nodemask, releasing both, taking them again, changing the
nodemask again to be disjoint, then the thread A waking up and finishing
its read and seeing an intersecting nodemask because it is now disjoint
from the first read?


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