Date: Wed, 23 Feb 2011 13:40:37 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
To: Peter Zijlstra <peterz@infradead.org>
cc: Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org, josh@joshtriplett.org,
        niv@us.ibm.com, tglx@linutronix.de, Valdis.Kletnieks@vt.edu,
        dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com,
        "Paul E. McKenney" <paul.mckenney@linaro.org>
Subject: Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE_RCU from softirq
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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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