From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753276AbXJaB6J (ORCPT ); Tue, 30 Oct 2007 21:58:09 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752440AbXJaB54 (ORCPT ); Tue, 30 Oct 2007 21:57:56 -0400 Received: from tomts5-srv.bellnexxia.net ([209.226.175.25]:61850 "EHLO tomts5-srv.bellnexxia.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752420AbXJaB5z (ORCPT ); Tue, 30 Oct 2007 21:57:55 -0400 X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: Ah4FAAZ+J0dMQWvU/2dsb2JhbACBWg Date: Tue, 30 Oct 2007 21:52:51 -0400 From: Mathieu Desnoyers To: Andrew Morton Cc: matthew@wil.cx, linux-kernel@vger.kernel.org, linux-mm@kvack.org, penberg@cs.helsinki.fi, linux-arch@vger.kernel.org, Christoph Lameter Subject: [PATCH] local_t Documentation update 2 Message-ID: <20071031015250.GA884@Krystal> References: <20071028033156.022983073@sgi.com> <20071028033300.240703208@sgi.com> <20071030114933.904a4cf8.akpm@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.6.21.3-grsec (i686) X-Uptime: 21:51:30 up 93 days, 2:10, 4 users, load average: 0.05, 0.26, 0.44 User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org local_t Documentation update 2 (this patch seems to have fallen off the grid, but is still providing useful information. It applies to 2.6.23-mm1.) Grant Grundler was asking for more detail about correct usage of local atomic operations and suggested adding the resulting summary to local_ops.txt. "Please add a bit more detail. If DaveM is correct (he normally is), then there must be limits on how the local_t can be used in the kernel process and interrupt contexts. I'd like those rules spelled out very clearly since it's easy to get wrong and tracking down such a bug is quite painful." Signed-off-by: Mathieu Desnoyers Signed-off-by: Grant Grundler --- Documentation/local_ops.txt | 23 +++++++++++++++++++++++ 1 file changed, 23 insertions(+) Index: linux-2.6-lttng/Documentation/local_ops.txt =================================================================== --- linux-2.6-lttng.orig/Documentation/local_ops.txt 2007-09-04 11:53:23.000000000 -0400 +++ linux-2.6-lttng/Documentation/local_ops.txt 2007-09-04 12:19:31.000000000 -0400 @@ -68,6 +68,29 @@ typedef struct { atomic_long_t a; } loca variable can be read when reading some _other_ cpu's variables. +* Rules to follow when using local atomic operations + +- Variables touched by local ops must be per cpu variables. +- _Only_ the CPU owner of these variables must write to them. +- This CPU can use local ops from any context (process, irq, softirq, nmi, ...) + to update its local_t variables. +- Preemption (or interrupts) must be disabled when using local ops in + process context to make sure the process won't be migrated to a + different CPU between getting the per-cpu variable and doing the + actual local op. +- When using local ops in interrupt context, no special care must be + taken on a mainline kernel, since they will run on the local CPU with + preemption already disabled. I suggest, however, to explicitly + disable preemption anyway to make sure it will still work correctly on + -rt kernels. +- Reading the local cpu variable will provide the current copy of the + variable. +- Reads of these variables can be done from any CPU, because updates to + "long", aligned, variables are always atomic. Since no memory + synchronization is done by the writer CPU, an outdated copy of the + variable can be read when reading some _other_ cpu's variables. + + * How to use local atomic operations #include -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68