Re: [PATCH] mm/mempolicy: fix sleeping function called from invalid context

David Rientjes
2014-06-10T22:16:10Z

On Tue, 10 Jun 2014, Li Zefan wrote:

> > Yes, the rcu lock is not providing protection for any critical section
> > here that requires (1) the forker's cpuset to be stored in
> > cpuset_being_rebound or (2) the forked thread's cpuset to be rebound by
> > the cpuset nodemask update, and no race involving the two.
> >
>
> Yes, this is a long-standing issue. Besides the race you described, the child
> task's mems_allowed can be wrong if the cpuset's nodemask changes before the
> child has been added to the cgroup's tasklist.
>
> I remember Tejun once said he wanted to disallow task migration between
> cgroups during fork, and that should fix this problem.
>

Ok, I don't want to fix it in cpusets if cgroups will eventually prevent
it, so I need an understanding of the long term plan. Will cgroups
continue to allow migration during fork(), Tejun?

> > It needs to be slightly rewritten to work properly without negatively
> > impacting the latency of fork(). Do you have the cycles to do it?
> >
>
> Sounds you have other idea?
>

It wouldn't be too difficult with a cgroup post fork callback into the
cpuset code to rebind the nodemask if it has changed, but with my above
concern those might be yanked out eventually :)