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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=unavailable 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 9D79EC433E0 for ; Mon, 25 May 2020 09:56:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8423D20787 for ; Mon, 25 May 2020 09:56:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389680AbgEYJ4X (ORCPT ); Mon, 25 May 2020 05:56:23 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59882 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389333AbgEYJ4X (ORCPT ); Mon, 25 May 2020 05:56:23 -0400 Received: from Galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4F188C061A0E; Mon, 25 May 2020 02:56:23 -0700 (PDT) Received: from [5.158.153.53] (helo=debian-buster-darwi.lab.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:RSA_AES_256_CBC_SHA1:256) (Exim 4.80) (envelope-from ) id 1jd9qA-0006Ec-Sg; Mon, 25 May 2020 11:56:15 +0200 Date: Mon, 25 May 2020 11:56:13 +0200 From: "Ahmed S. Darwish" To: Peter Zijlstra Cc: Ingo Molnar , Will Deacon , Thomas Gleixner , "Paul E. McKenney" , "Sebastian A. Siewior" , Steven Rostedt , LKML , Jens Axboe , Phillip Susi , Vivek Goyal , linux-block@vger.kernel.org Subject: Re: [PATCH v1 04/25] block: nr_sects_write(): Disable preemption on seqcount write Message-ID: <20200525095613.GB370823@debian-buster-darwi.lab.linutronix.de> References: <20200519214547.352050-1-a.darwish@linutronix.de> <20200519214547.352050-5-a.darwish@linutronix.de> <20200522163908.GP325280@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200522163908.GP325280@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.1 (2018-07-13) X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Peter Zijlstra wrote: > On Tue, May 19, 2020 at 11:45:26PM +0200, Ahmed S. Darwish wrote: > > For optimized block readers not holding a mutex, the "number of sectors" > > 64-bit value is protected from tearing on 32-bit architectures by a > > sequence counter. > > > > Disable preemption before entering that sequence counter's write side > > critical section. Otherwise, the read side can preempt the write side > > section and spin for the entire scheduler tick. If the reader belongs to > > a real-time scheduling class, it can spin forever and the kernel will > > livelock. > > > > Fixes: c83f6bf98dc1 ("block: add partition resize function to blkpg ioctl") > > Cc: > > Signed-off-by: Ahmed S. Darwish > > Reviewed-by: Sebastian Andrzej Siewior > > --- > > block/blk.h | 2 ++ > > 1 file changed, 2 insertions(+) > > > > diff --git a/block/blk.h b/block/blk.h > > index 0a94ec68af32..151f86932547 100644 > > --- a/block/blk.h > > +++ b/block/blk.h > > @@ -470,9 +470,11 @@ static inline sector_t part_nr_sects_read(struct hd_struct *part) > > static inline void part_nr_sects_write(struct hd_struct *part, sector_t size) > > { > > #if BITS_PER_LONG==32 && defined(CONFIG_SMP) > > + preempt_disable(); > > write_seqcount_begin(&part->nr_sects_seq); > > part->nr_sects = size; > > write_seqcount_end(&part->nr_sects_seq); > > + preempt_enable(); > > #elif BITS_PER_LONG==32 && defined(CONFIG_PREEMPTION) > > preempt_disable(); > > part->nr_sects = size; > > This does look like something that include/linux/u64_stats_sync.h could > help with. Correct. I just felt though that this would be too much for a 'Cc: stable' patch. In another (in-progress) seqlock.h patch series, all of the seqcount_t call sites that are used for 64-bit values tearing protection on 32-bit kernels are transformed to the u64_stats_sync.h API. Thanks, -- Ahmed S. Darwish Linutronix GmbH