From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 861A2CE79A9 for ; Tue, 19 Sep 2023 15:58:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233232AbjISP6E convert rfc822-to-8bit (ORCPT ); Tue, 19 Sep 2023 11:58:04 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54130 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233230AbjISP6B (ORCPT ); Tue, 19 Sep 2023 11:58:01 -0400 Received: from eu-smtp-delivery-151.mimecast.com (eu-smtp-delivery-151.mimecast.com [185.58.85.151]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2A16593 for ; Tue, 19 Sep 2023 08:57:54 -0700 (PDT) Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) by relay.mimecast.com with ESMTP with both STARTTLS and AUTH (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id uk-mta-232-by0VL1p0PYKojar-QhGBLQ-1; Tue, 19 Sep 2023 16:57:41 +0100 X-MC-Unique: by0VL1p0PYKojar-QhGBLQ-1 Received: from AcuMS.Aculab.com (10.202.163.6) by AcuMS.aculab.com (10.202.163.6) with Microsoft SMTP Server (TLS) id 15.0.1497.48; Tue, 19 Sep 2023 16:57:36 +0100 Received: from AcuMS.Aculab.com ([::1]) by AcuMS.aculab.com ([::1]) with mapi id 15.00.1497.048; Tue, 19 Sep 2023 16:57:36 +0100 From: David Laight To: 'Matthew Wilcox' CC: Greg Ungerer , "linux-kernel@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-arch@vger.kernel.org" , "torvalds@linux-foundation.org" , Nicholas Piggin Subject: RE: [PATCH 09/17] m68k: Implement xor_unlock_is_negative_byte Thread-Topic: [PATCH 09/17] m68k: Implement xor_unlock_is_negative_byte Thread-Index: AQHZ6Kr+hLdybFnA00ugx5L/MFgM8bAiJxPQgAABTgCAABHZMP//+8IAgAARftD///eTgIAAEctA Date: Tue, 19 Sep 2023 15:57:36 +0000 Message-ID: References: <20230915183707.2707298-1-willy@infradead.org> <20230915183707.2707298-10-willy@infradead.org> <6e409d5f-a419-07b7-c82c-4e80fe19c6ba@westnet.com.au> In-Reply-To: Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org From: Matthew Wilcox > Sent: 19 September 2023 16:47 > > On Tue, Sep 19, 2023 at 03:22:25PM +0000, David Laight wrote: > > > Anyway, that's not the brief. We're looking to (eg) clear bit 0 > > > and test whether bit 7 was set. So it's the sign bit of the byte, > > > not the sign bit of the int. > > > > Use the address of the byte as an int and xor with 1u<<24. > > The xor will do a rmw on the three bytes following, but I > > doubt that matters. > > Bet you a shiny penny that Coldfire takes an unaligned access trap ... and then the 'firmware' silently fixed it up for you a few 1000 clocks later... > and besides, this is done on _every_ call to unlock_page(). That might > cross not only a cacheline boundary but also a page boundary. I cannot > believe that would be a high-performing solution. It might be just fine > on m68000 but I bet even by the 030 it's lower performing. I do remember managing to use 'cas2' to add an item to a linked list. But it is so painful so setup it was better just to disable interrupts. For non-smp that is almost certainly ok. (Unless the instructions are slow because of synchronisation.) Otherwise you need to use 'cas' on the aligned word. Assuming coldfire even has cas. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)