linux-kernel - Re: [PATCH v15 04/13] task_isolation: add initial support

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Date:   Tue, 27 Sep 2016 16:51:29 +0200
From:   Frederic Weisbecker <fweisbec@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        Christoph Lameter <cl@...ux.com>,
        Michal Hocko <mhocko@...e.com>,
        Gilad Ben Yossef <giladb@...lanox.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Linux API <linux-api@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>, Tejun Heo <tj@...nel.org>,
        Rik van Riel <riel@...hat.com>,
        Will Deacon <will.deacon@....com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH v15 04/13] task_isolation: add initial support

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!