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 X-Spam-Level: X-Spam-Status: No, score=-3.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DE02EC4361B for ; Mon, 7 Dec 2020 16:48:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9F5BB23884 for ; Mon, 7 Dec 2020 16:48:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727938AbgLGQsU convert rfc822-to-8bit (ORCPT ); Mon, 7 Dec 2020 11:48:20 -0500 Received: from eu-smtp-delivery-151.mimecast.com ([207.82.80.151]:33219 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727303AbgLGQsU (ORCPT ); Mon, 7 Dec 2020 11:48:20 -0500 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-242-QI3yR-scPsyRFFaCGeCl_A-1; Mon, 07 Dec 2020 16:46:41 +0000 X-MC-Unique: QI3yR-scPsyRFFaCGeCl_A-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Mon, 7 Dec 2020 16:46:40 +0000 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Mon, 7 Dec 2020 16:46:40 +0000 From: David Laight To: 'John Ogness' , Peter Zijlstra CC: Petr Mladek , Sergey Senozhatsky , Sergey Senozhatsky , Steven Rostedt , "Linus Torvalds" , Greg Kroah-Hartman , Thomas Gleixner , "linux-kernel@vger.kernel.org" Subject: RE: [PATCH next v2 2/3] printk: change @clear_seq to atomic64_t Thread-Topic: [PATCH next v2 2/3] printk: change @clear_seq to atomic64_t Thread-Index: AQHWzICKswdVD7bXr0KkK6JFsMrIzKnr1zJA Date: Mon, 7 Dec 2020 16:46:40 +0000 Message-ID: References: <20201201205341.3871-1-john.ogness@linutronix.de> <20201201205341.3871-3-john.ogness@linutronix.de> <875z5eof8g.fsf@jogness.linutronix.de> <20201207093419.GH3040@hirez.programming.kicks-ass.net> <87mtyq9blw.fsf@jogness.linutronix.de> In-Reply-To: <87mtyq9blw.fsf@jogness.linutronix.de> 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 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com 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-kernel@vger.kernel.org From: John Ogness > Sent: 07 December 2020 10:04 > > On 2020-12-07, Peter Zijlstra wrote: > >> Yes, and it is read-only access. Perhaps atomic64_t is the wrong thing > >> to use here. We could use a seqcount_latch and a shadow variable so that > >> if a writer has been preempted, we can use the previous value. (Only > >> kmsg_dump would need to use the lockless variant to read the value.) > >> > >> void clear_seq_set(u64 val) > >> { > >> spin_lock_irq(&clear_lock); > >> raw_write_seqcount_latch(&clear_latch); > >> clear_seq[0] = val; > >> raw_write_seqcount_latch(&clear_latch); > >> clear_seq[1] = val; > >> spin_unlock_irq(&clear_lock); > >> } > >> > >> u64 clear_seq_get_nolock(void) > >> { > >> unsigned int seq, idx; > >> u64 val; > >> > >> do { > >> seq = raw_read_seqcount_latch(&clear_latch); > >> idx = seq & 0x1; > >> val = clear_seq[idx]; > >> } while (read_seqcount_latch_retry(&clear_latch, seq)); > >> > >> return val; > >> } > > > > That's overly complicated. > > > > If you're going to double the storage you can simply do: > > > > > > seq = val > > smp_wmb(); > > seq_copy = val; > > > > vs > > > > do { > > tmp = seq_copy; > > smp_rmb(); > > val = seq; > > } while (val != tmp); > > That will not work. We are talking about a situation where the writer is > preempted. So seq will never equal seq_copy in that situation. I expect > that the seqcount_latch is necessary. Is the value just being incremented?? If so you can do: seq_hi_0 = val >> 32; smp_wmb(); seq_lo = val; smp_wmb(); seq_hi_1 = val >> 32; Then the reader can assume that seq_lo is zero if seq_h1_0 and seq_hi_1 differ. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)