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

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Message-Id: <20180622205859.GW3593@linux.vnet.ibm.com>
Date:   Fri, 22 Jun 2018 13:58:59 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Byungchul Park <max.byungchul.park@...il.com>,
        Byungchul Park <byungchul.park@....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 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.

There was.  Been there, got the failure on real hardware.  ;-)

							Thanx, Paul