linux-kernel - Re: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct rcu

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Message-ID: <20180625163951.GA52646@joelaf.mtv.corp.google.com>
Date:   Mon, 25 Jun 2018 09:39:51 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Byungchul Park <max.byungchul.park@...il.com>,
        jiangshanlai@...il.com, josh@...htriplett.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        linux-kernel@...r.kernel.org, kernel-team@....com, luto@...nel.org
Subject: Re: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct
 rcu_dynticks

On Mon, Jun 25, 2018 at 05:28:24PM +0900, Byungchul Park wrote:
> On Fri, Jun 22, 2018 at 01:05:48PM -0700, Joel Fernandes wrote:
> > On Fri, Jun 22, 2018 at 02:32:47PM -0400, Steven Rostedt wrote:
> > > On Fri, 22 Jun 2018 11:19:16 -0700
> > > Joel Fernandes <joel@...lfernandes.org> wrote:
> > > 
> > > > Sure. So in a later thread you mentioned "usermode helpers". I took a closer
> > > > look at that subsystem, and it seems you can execute usermode helpers from
> > > > atomic sections with help of UMH_NO_WAIT flag.
> > > > 
> > > > Then I checked where this flag is used and it turns out its from the
> > > > mce_work_trigger function in x86/kernel/cpu/mcheck/dev-mcelog.c which can be
> > > > called infact from an interrupt context (mce_notify_irq).
> > > > 
> > > > Is this the usecase you remember causing this weird transitions to userspace?
> > > 
> > > But this case still looks like it uses work queues, it just doesn't
> > > wait for the result.
> > > 
> > > I'll have to look at the code from what it looked like back in 2011, to
> > > see if there was an actual issue here back then.
> > 
> > Good point Steve. So I guess in the current kernel sources, there's no code
> > that uses UMH in IRQ context AFAICT. I'll go through the google group thread
> > Paul pointed as well to study the history of the problem a bit more.
> 
> Me too. Good discussion we had thanks to you, Joel.

No problem, thanks for the patch in the first place which triggered this
discussion.

For whatever its worth, I made some notes of what I understood from reading
the code and old posts because I was sure I would otherwise forget
everything:
http://www.joelfernandes.org/linuxinternals/2018/06/15/rcu-dynticks.html

Feel free to comment on that post directly (or here) if you feel something is
grossly wrong.

Again thank you and everyone for the discussion! ;-)

 - Joel