Re: [PATCH 1/4] rcu: Allow for page faults in NMI handlers

Paul E. McKenney
2017-09-26T03:19:10Z

On Sun, Sep 24, 2017 at 09:56:32PM -0700, Paul E. McKenney wrote:
> On Mon, Sep 25, 2017 at 06:41:30AM +0200, Steven Rostedt wrote:
> > Sorry for the top post, currently on a train to Paris.
> >
> > This series already went through all my testing, and I would hate to rebase it for this reason. Can you just add a patch to remove the READ_ONCE()s?
>
> If Linus accepts the original series, easy enough.

And he did, so here is the READ_ONCE()-removal commit that I have queued
for the next merge window. If anyone feels it is needed sooner, please
let me know. (Can't see why it is urgent myself, but who knows...)

Thanx, Paul

------------------------------------------------------------------------

commit 79e6337d3f3a629be48cd45d5075d058788ce90f
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date: Mon Sep 25 20:07:49 2017 -0700

rcu: Remove extraneous READ_ONCE()s from rcu_irq_{enter,exit}()

The read of ->dynticks_nmi_nesting in rcu_irq_enter() and rcu_irq_exit()
is currently protected with READ_ONCE(). However, this protection is
unnecessary because (1) ->dynticks_nmi_nesting is updated only by the
current CPU, (2) Although NMI handlers can update this field, they reset
it back to its old value before return, and (3) Interrupts are disabled,
so nothing else can modify it. The value of ->dynticks_nmi_nesting is
thus effectively constant, and so no protection is required.

This commit therefore removes the READ_ONCE() protection from these
two accesses.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 872d20cee00a..e4fe06d42385 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -893,7 +893,7 @@ void rcu_irq_exit(void)
rdtp = this_cpu_ptr(&rcu_dynticks);

/* Page faults can happen in NMI handlers, so check... */
- if (READ_ONCE(rdtp->dynticks_nmi_nesting))
+ if (rdtp->dynticks_nmi_nesting)
return;

WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
@@ -1043,7 +1043,7 @@ void rcu_irq_enter(void)
rdtp = this_cpu_ptr(&rcu_dynticks);

/* Page faults can happen in NMI handlers, so check... */
- if (READ_ONCE(rdtp->dynticks_nmi_nesting))
+ if (rdtp->dynticks_nmi_nesting)
return;

oldval = rdtp->dynticks_nesting;