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Date:   Fri, 22 Jun 2018 13:58:59 -0700
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        Byungchul Park <max.byungchul.park@gmail.com>,
        Byungchul Park <byungchul.park@lge.com>,
        jiangshanlai@gmail.com, josh@joshtriplett.org,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        linux-kernel@vger.kernel.org, kernel-team@lge.com, luto@kernel.org
Subject: Re: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct
 rcu_dynticks
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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@joelfernandes.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