linux-kernel - Re: [PATCH] sched/numa: fix unsafe get_task_struct() in task_numa

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Message-ID: <20141020091311.GC3219@twins.programming.kicks-ass.net>
Date:	Mon, 20 Oct 2014 11:13:11 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Kirill Tkhai <tkhai@...dex.ru>,
	Kirill Tkhai <ktkhai@...allels.com>,
	"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()


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.
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