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

[Peter Zijlstra <peterz@infradead.org>]

OK, I think I'm finally awake enough to see what you're all talking
about :-)

On Sun, Oct 19, 2014 at 09:37:44PM +0200, Oleg Nesterov wrote:
> > > RT tree has:
> > >
> > > https://git.kernel.org/cgit/linux/kernel/git/paulg/3.10-rt-patches.git/
> > > tree/patches/sched-delay-put-task.patch

(answering the other email asking about this)

RT does this because we call put_task_struct() with preempt disabled and
on RT the memory allocator has sleeping locks.

> > Yes, and this obviously implies more rcu callbacks in flight, and another
> > gp before __put_task_struct(). but may be we will need to do this anyway...
> 
> Forgot to mention... Or we can make task_struct_cachep SLAB_DESTROY_BY_RCU,
> in this case ->curr (or any other "task_struct *" ponter) can not go away
> under rcu_read_lock(). task_numa_compare() still needs the PF_EXITING check,
> but we do not need to recheck ->curr or probe_kernel_read().

I think I would prefer SLAB_DESTROY_BY_RCU for this, because as you
pointed out, I'm not sure mainline would like the extra callbacks.