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Date: Thu, 13 Dec 2012 15:00:15 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Gilad Ben-Yossef <gilad@benyossef.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Li Zhong <zhong@linux.vnet.ibm.com>
Subject: Re: [PATCH] context_tracking: Add comments on interface and
 internals
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On Thu, 13 Dec 2012 23:50:23 +0100
Frederic Weisbecker <fweisbec@gmail.com> wrote:

> >
> >> + * This call supports re-entrancy.
> >
> > Presumably the explanation for user_exit() applies here.
> 
> Not sure what you mean here.

It's unclear what it means to say "user_enter() supports reentrancy". 
I mean, zillions of kernel functions are surely reentrant - so what? 
It appears that you had something in mind when pointing this out, but
what was it?  The comment over user_exit() appears to tell us.

> > It's mainly this bit which makes me wonder why the code is in lib/.  Is
> > there any conceivable prospect that any other subsystem will use this
> > code for anything?
> 
> So that's because of that cputime accounting on dynticks CPUs which
> will need to know about user/kernel transitions. I'm preparing that
> for the 3.9 merge window.

Oh.  That's really the entire reason for the patch and should have been
in the changelog!