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From: Rusty Russell <rusty@rustcorp.com.au>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Subject: Re: local_add_return
Date: Fri, 19 Dec 2008 09:22:56 +1030
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Cc: David Miller <davem@davemloft.net>, rostedt@goodmis.org,
       akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
       paulus@samba.org, benh@kernel.crashing.org, linux-ia64@vger.kernel.org,
       linux-s390@vger.kernel.org, Christoph Lameter <christoph@lameter.com>,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Martin Bligh <mbligh@google.com>
References: <alpine.DEB.1.10.0812150823370.18692@gandalf.stny.rr.com> <200812170908.05423.rusty@rustcorp.com.au> <20081217000155.GA28174@Krystal>
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On Wednesday 17 December 2008 10:31:55 Mathieu Desnoyers wrote:
> I think we have two different use-cases here :
> 
> - local_t is useful as-is for things such as a tracer, which need to
>   modify an element of data atomically wrt local interrupts. The
>   atomic_long_t, in this case, is the correct fallback.
> - local_count_t could be used for fast counters.

Hi Mathieu,

   Complete agreement.

   I guess I'm biassed towards local_t == counter version, something else
== nmi-safe version because that's what it was originally.  Looking through
the tree, there are only 5 users: module, dmaengine and percpu_counter want
a counter, and tracing and x86 nmi.c want nmi-safe.  There are several other
places I know of which want local_t-the-counter.

   I'll prepare a patch which adds nmi_safe_t, and see how it looks.  There's
no amazing hurry on this, so I won't race to hit the merge window.

Thanks!
Rusty.