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Date:	Mon, 27 Oct 2014 20:53:39 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Kirill Tkhai <ktkhai@...allels.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Kirill Tkhai <tkhai@...dex.ru>,
	Christoph Lameter <cl@...ux.com>
Subject: [PATCH 0/3] introduce task_rcu_dereference()

Peter, let me repeat once again, if you still prefer to avoid
probe_slab_address() and use SLAB_DESTROY_BY_RCU I won't argue.

I do not like SLAB_DESTROY_BY_RCU in this particular case. With
or without SDBR rq->curr can be reused and we need to avoid this
race. The fact that with SDBR it can be reused only as another
instance of task_struct is absolutely immaterial imo.

Not to mention that SDBR still adds some overhead while probe_slab()
is free unless CONFIG_DEBUG_PAGEALLOC, but this option adds a large
slowdown anyway.

However, my arguments against SDBR are not strictly technical, and
I think that this falls into "maintainer is always right" category.

So please tell me if you prefer v2 with SDBR. In this case 2/3 is
not needed, and 3/3 can simply read ->sighand. Otherwise the code
(and even the comments) will be the same.

Compared to the draft patch I sent before

	- update the comments

	- do not use ERR_PTR(), just return the task or NULL, so
	  kernel/sched/ doesn't need another helper.

	  This means that task_rcu_dereference() does retry itself.
	  We can add __task_rcu_dereference() if we have another
	  which do not need/want to retry.

Oleg.

 include/linux/sched.h   |    1 +
 include/linux/uaccess.h |   30 +++++++++++++++-------------
 kernel/exit.c           |   49 +++++++++++++++++++++++++++++++++++++++++++++++
 mm/slub.c               |    6 +----
 4 files changed, 67 insertions(+), 19 deletions(-)

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