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SubjectRe: [PATCH] task containersv11 add tasks file interface fix for cpusets
On Sat, 6 Oct 2007, Paul Jackson wrote:

> This isn't working for me.
>
> The key kernel routine for updating a tasks cpus_allowed
> cannot be called while holding a spinlock.
>
> But the above loop holds a spinlock, css_set_lock, between
> the cgroup_iter_start and the cgroup_iter_end.
>
> I end up generating complaints of:
> BUG: scheduling while atomic
> when I invoke the set_cpus_allowed() above.
>
> Should css_set_lock be a mutex? Locking changes like that
> can be risky.
>

It would probably be better to just save references to the tasks.

struct cgroup_iter it;
struct task_struct *p, **tasks;
int i = 0;

cgroup_iter_start(cs->css.cgroup, &it);
while ((p = cgroup_iter_next(cs->css.cgroup, &it))) {
get_task_struct(p);
tasks[i++] = p;
}
cgroup_iter_end(cs->css.cgroup, &it);
while (--i >= 0) {
set_cpus_allowed(tasks[i], cs->cpus_allowed);
put_task_struct(tasks[i]);
}
The getting and putting of the tasks will prevent them from exiting or
being deallocated prematurely. But this is also a critical section that
will need to be protected by some mutex so it doesn't race with other
set_cpus_allowed().

David
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