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Date:   Fri, 30 Aug 2019 18:44:37 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Russell King - ARM Linux admin <linux@armlinux.org.uk>,
        Peter Zijlstra <peterz@infradead.org>,
        Chris Metcalf <cmetcalf@ezchip.com>,
        Christoph Lameter <cl@linux.com>,
        Kirill Tkhai <tkhai@yandex.ru>, Mike Galbraith <efault@gmx.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>
Subject: Re: [BUG] Use of probe_kernel_address() in task_rcu_dereference()
 without checking return value
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On 08/30, Linus Torvalds wrote:
>
> But yes, your hack is I guess optimal for this particular case where
> you simply can depend on "we know the pointer was valid, we just don't
> know if it was freed".
>
> Hmm. Don't we RCU-free the task struct? Because then we don't even
> need to care about CONFIG_DEBUG_PAGEALLOC. We can just always access
> the pointer as long as we have the RCU read lock.

For example,

	rcu_read_lock();
	p = task_rcu_dereference(&cpu_rq(cpu)->curr);
	rcu_read_unlock();

->curr is not protected by RCU, the last schedule does put_task_struct()
in finish_task_switch().

Of course we can change this and add another call_rcu (actually we can do
better), and after that we do not need task_rcu_dereference() at all.

Oleg.