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

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Date:   Fri, 22 Jun 2018 12:01:49 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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, 22 Jun 2018 06:28:43 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> It has been some years since I traced the code flow, but what happened
> back then is that it switches itself from an interrupt handler to not
> without actually returning from the interrupt.  This can only happen when
> interrupting a non-idle process, thankfully, and RCU's dyntick-idle code
> relies on this restriction.  If I remember correctly, the code ends up
> executing in the context of the interrupted process, but it has been some
> years, so please apply appropriate skepticism.

If irq_enter() is not paired with irq_exit() then major things will
break. Especially since that's how in_interrupt() and friends rely on to
work.

Now, perhaps rcu_irq_enter() is called elsewhere (as a git grep appears
it may be), and that rcu_irq_enter() may not be paired with
rcu_irq_exit(). But that's not anything to do with the irq_enter() and
irq_exit() routines being paired or not.

-- Steve