Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756147AbXH1Gjb (ORCPT ); Tue, 28 Aug 2007 02:39:31 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750952AbXH1GjX (ORCPT ); Tue, 28 Aug 2007 02:39:23 -0400 Received: from colo.lackof.org ([198.49.126.79]:38932 "EHLO colo.lackof.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751456AbXH1GjW (ORCPT ); Tue, 28 Aug 2007 02:39:22 -0400 Date: Tue, 28 Aug 2007 00:39:05 -0600 From: Grant Grundler To: Mathieu Desnoyers Cc: Grant Grundler , akpm@linux-foundation.org, linux-kernel@vger.kernel.org, parisc-linux@parisc-linux.org, Christoph Lameter Subject: Re: [parisc-linux] [patch 15/23] Add cmpxchg_local to parisc Message-ID: <20070828063905.GA3916@colo.lackof.org> References: <20070812145434.520271946@polymtl.ca> <20070812145840.691277845@polymtl.ca> <20070827210432.GA22484@colo.lackof.org> <20070827211140.GB10627@Krystal> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20070827211140.GB10627@Krystal> X-Home-Page: http://www.parisc-linux.org/ User-Agent: Mutt/1.5.16 (2007-06-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2814 Lines: 70 On Mon, Aug 27, 2007 at 05:11:40PM -0400, Mathieu Desnoyers wrote: ... > > Can you add a description to Documentation/atomic_ops.txt ? > > *sigh* sorry for being "late to the party" on this one... > > Does Documentation/local_ops.txt answer your questions ? If not, please > tell me and I'll gladly explain more. Yes, it does mostly - thanks. A few questions/nits: o Did you attempt quantify how many places in the kernel could use this? I'm just trying to get a feel for how useful this really is vs just using existing mechanisms (that people understand) to implement a non-SMP-safe counter that protects updates (writes) against interrupts. If you did, adding some referencs to local_ops.txt would be helpful so folks could look for examples of "correct usage". o Wording in local_ops.txt: "on the "... it will then appear to be written out of order wrt other memory writes on the owner CPU." I'd like to suggest "by the owner CPU". o How can a local_t counter protect updates (writes) against interrupts but not preemption? I always thought preemption required some sort of interrupt or trap. Maybe the local_ops.txt explains that and I just missed it. DaveM explained updates "in flight" would not be visible to interrupts and I suspect that's the answer to my question....but then I don't "feel good" the local_ops are safe to update in interrupts _and_ the process context kernel. Maybe the relationship between local_ops, preemption, and interrupts could be explained more carefully in local_ops.txt. o OK to add a reference for local_ops.txt to atomic_ops.txt? They are obviously related and anyone "discovering" one of the docs should be made aware of the other. Patch+log entry appended below. Please sign-off if that's ok with you. thanks, grant Diff+Commit entry against 2.6.22.5: local_t is a variant of atomic_t and has related ops to match. Add reference for local_t documentation to atomic_ops.txt. Signed-off-by: Grant Grundler --- 2.6.22.5-ORIG/Documentation/atomic_ops.txt 2007-08-27 22:50:27.000000000 -0700 +++ 2.6.22.5-ggg/Documentation/atomic_ops.txt 2007-08-27 22:54:44.000000000 -0700 @@ -14,6 +14,10 @@ typedef struct { volatile int counter; } atomic_t; +local_t is very similar to atomic_t. If the counter is per CPU and only +updated by one CPU, local_t is probably more appropriate. Please see +Documentation/local_ops.txt for the semantics of local_t. + The first operations to implement for atomic_t's are the initializers and plain reads. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/