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Date: Tue, 27 Sep 2016 16:51:29 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@amacapital.net>,
        Chris Metcalf <cmetcalf@mellanox.com>,
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        Will Deacon <will.deacon@arm.com>,
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Subject: Re: [PATCH v15 04/13] task_isolation: add initial support
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On Tue, Sep 27, 2016 at 04:39:26PM +0200, Peter Zijlstra wrote:
> On Tue, Sep 27, 2016 at 04:22:20PM +0200, Frederic Weisbecker wrote:
> 
> > The RCU context tracking doesn't take care of callbacks. It's only there
> > to tell the RCU core whether the CPU runs code that may or may not run
> > RCU read side critical sections. This is assumed by "kernel may use RCU,
> > userspace can't".
> 
> Userspace never can use the kernels RCU in any case. What you mean to
> say is that userspace is treated like an idle CPU in that the CPU will
> no longer be part of the RCU quescent state machine.
> 
> The transition to userspace (as per context tracking) must ensure that
> CPUs RCU state is 'complete', just like our transition to idle (mostly)
> does.

Exactly!