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

David Rientjes
2014-06-09T09:14:07Z

On Mon, 9 Jun 2014, Gu Zheng wrote:

> > I think your patch addresses the problem that you're reporting but misses
> > the larger problem with cpuset.mems rebinding on fork(). When the
> > forker's task_struct is duplicated (which includes ->mems_allowed) and it
> > races with an update to cpuset_being_rebound in update_tasks_nodemask()
> > then the task's mems_allowed doesn't get updated.
>
> Yes, you are right, this patch just wants to address the bug reported above.
> The race condition you mentioned above inherently exists there, but it is yet
> another issue, the rcu lock here makes no sense to it, and I think we need
> additional sync-mechanisms if want to fix it.

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.

> But thinking more, though the current implementation has flaw, but I worry
> about the negative effect if we really want to fix it. Or maybe the fear
> is unnecessary.:)
>

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