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Date: Tue, 27 Sep 2016 16:39:26 +0200
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v15 04/13] task_isolation: add initial support
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To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>, Chris Metcalf <cmetcalf@mellanox.com>, Thomas Gleixner <tglx@linutronix.de>, "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>, Christoph Lameter <cl@linux.com>, Michal Hocko <mhocko@suse.com>, Gilad Ben Yossef <giladb@mellanox.com>, Andrew Morton <akpm@linux-foundation.org>, Viresh Kumar <viresh.kumar@linaro.org>, Linux API <linux-api@vger.kernel.org>, Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@kernel.org>, Tejun Heo <tj@kernel.org>, Rik van Riel <riel@redhat.com>, Will Deacon <will.deacon@arm.com>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Catalin Marinas <catalin.marinas@arm.com>

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.

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