linux-kernel - Re: local_add

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Message-Id: <200812190922.57629.rusty@rustcorp.com.au>
Date:	Fri, 19 Dec 2008 09:22:56 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	David Miller <davem@...emloft.net>, rostedt@...dmis.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	paulus@...ba.org, benh@...nel.crashing.org,
	linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
	Christoph Lameter <christoph@...eter.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Martin Bligh <mbligh@...gle.com>
Subject: Re: local_add_return

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.
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