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Date:	Mon, 20 Oct 2014 14:36:13 +0400
From:	Kirill Tkhai <ktkhai@...allels.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Oleg Nesterov <oleg@...hat.com>, Kirill Tkhai <tkhai@...dex.ru>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	"Vladimir Davydov" <vdavydov@...allels.com>
Subject: Re: [PATCH] sched/numa: fix unsafe get_task_struct() in
 task_numa_assign()

В Пн, 20/10/2014 в 11:13 +0200, Peter Zijlstra пишет:
> 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.

Now it's clearly for me. I though it's because task_struct freeing is slow.
Thanks!

> > > 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.

I've sent one more patch with this:

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

Kirill

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