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

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Date:   Mon, 25 Jun 2018 17:28:24 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Joel Fernandes <joel@...lfernandes.org>
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 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.

> thanks,
> 
> - Joel