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Date: Tue, 10 Mar 2015 18:28:16 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
        "linux-tip-commits@vger.kernel.org" 
	<linux-tip-commits@vger.kernel.org>,
        Davidlohr Bueso <dave@stgolabs.net>, Peter Anvin <hpa@zytor.com>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        Ingo Molnar <mingo@kernel.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Dave Jones <davej@codemonkey.org.uk>,
        Ming Lei <ming.lei@canonical.com>,
        Tim Chen <tim.c.chen@linux.intel.com>, Kirill Tkhai <tkhai@yandex.ru>
Subject: Re: [tip:locking/core] locking/rwsem: Fix lock optimistic spinning
	when owner is not running
Message-ID: <20150310172816.GA9058@redhat.com>
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On 03/10, Linus Torvalds wrote:
>
> On Sat, Mar 7, 2015 at 9:13 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> >> +             /*
> >> +              * Ensure we emit the owner->on_cpu, dereference _after_
> >> +              * checking sem->owner still matches owner, if that fails,
> >> +              * owner might point to free()d memory, if it still matches,
> >> +              * the rcu_read_lock() ensures the memory stays valid.
> >                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >
> > Yes, this is another case when we wrongly assume this.
> >
> > Peter, should I resend
> >
> >         [PATCH 3/3] introduce task_rcu_dereference()
> >         http://marc.info/?l=linux-kernel&m=141443631413914
> >
> > ? or should we add another call_rcu() in finish_task_switch() (like -rt does)
> > to make this true?
>
> I think we should just make 'task_struct_cachep' have SLAB_DESTROY_BY_RCU.

This is what I initially suggested too, but then tried to argue with.
But it seems that I lost if you too prefer SLAB_DESTROY_BY_RCU.

Yes, SLAB_DESTROY_BY_RCU will work in this case because we recheck
->owner in a loop. And because task->on_cpu is just a word we can
safely read.

But this won't fix other problems we might have. For example, suppose
that we will need get_task_struct(owner) in this code, this won't work.

Or, as Kirill pointed out, lets look at "tsk = ACCESS_ONCE(cpu_rq(cpu)->curr)"
in task_numa_group(). Even if this will be "fixed" by SLAB_DESTROY_BY_RCU,
this code won't be correct anyway. Even if (I think) it will be safe to
dereference ->numa_group as well.

But OK, I won't argue.

Oleg.