linux-kernel - Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE

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Date:	Wed, 23 Feb 2011 13:40:37 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE_RCU from softirq
 to kthread

On Wed, 23 Feb 2011, Peter Zijlstra wrote:

> On Wed, 2011-02-23 at 13:19 -0600, Christoph Lameter wrote:
>
> > this_cpu_inc is already percpu atomic. On x86 it is an instruction that
> > cannot be interrupted nor preempted while in progress.
>
> On x86, yes, but is this true for all architectures? I guess the
> fallback is implemented with local_irq_disable() which might be good
> enough for some but not for NMI usage.

Fallback is to do preempt_disable(). But you can also disable interrupts
on fallback by putting an irqsafe_ prefix before them. Its just more
expensive and typically not needed.

There is no way to create an nmi safe per cpu atomicity emulation as far
as I can tell. It is possible to check at compile time if the arch has
per cpu atomic instructions available via CONFIG_CMPXCHG_LOCAL which
allows optimized per cpu atomic use for arches that satisfy the safety
requirements.
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