Re: [PATCH v3] sched/numa: fix unsafe get_task_struct() in task_numa_assign()

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Oct 20, 2014 at 10:50:06PM +0200, Oleg Nesterov wrote:
> Let me explain what I personally dislike in v3:
> 
> 	- I think that we do not have enough reasons for
> 	  SLAB_DESTROY_BY_RCU. This is the serious change.

What exactly would the downsides be? SDBR has very limited space
overhead iirc.

> 	- Again, perhaps we should start we a simple and stupid fix.
> 	  We can do get_task_struct() under rq->lock or, if nothing
> 	  else, just
> 
> 	  	raw_spin_lock_irq(&rq->lock);
> 	  	cur = rq->curr;
> 	  	if (is_idle_task(cur) || (cur->flags & PF_EXITING))
> 	  		cur = NULL;
> 	  	raw_spin_unlock_irq(&rq->lock);

I think I agree with you, this is the simple safe option and is
something we can easily backport. After that we can add creative bits on
top.

I think I prefer the SLAB_DESTROY_BY_RCU thing over the probe_kernel
thing -- but we can take our time once we've fixed the immediate issue
with the simple option.