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=-6.5 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 19705C433E1 for ; Fri, 22 May 2020 16:39:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E9A172054F for ; Fri, 22 May 2020 16:39:20 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="SNa6ZJDj" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730510AbgEVQjU (ORCPT ); Fri, 22 May 2020 12:39:20 -0400 Received: from merlin.infradead.org ([205.233.59.134]:44600 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726762AbgEVQjT (ORCPT ); Fri, 22 May 2020 12:39:19 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=8OmNhX8P6hsaXiRZDKUBUMQjeMukKlzzGx4//zwIpCI=; b=SNa6ZJDj6oroJjon29xMbzAjGe QkkzzexMe6l9wA1sLWK8+fKpqOS3SXo7IJQc4JIBNh34NuGgpKMnlY74y+HLkikTciBq5zLRV71YT Ar/5PMNhsBxvLI41GHs3sK+l67eaH+gjwpQt8/sq9jbnn0oXMRdG9I4z+5mE821fe69SKjuESitjW X/6P7oMwCoiEXadoQSyWZb1q33EMeo1T73RpdvHhJ2BucC5XUkJ7qair5s/+ekDuWYZY4PlbLX7YR PId4sYnpxXKJez/i1HNU+qZvLtQfJpdXX8khVTn6slS4bzRiyqlnYcNvvzCbKLlEGG6oJuGz7VA5C 7s4ZPGrg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1jcAhU-0006It-Cd; Fri, 22 May 2020 16:39:12 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 41C16306102; Fri, 22 May 2020 18:39:09 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id F13E520BDB125; Fri, 22 May 2020 18:39:08 +0200 (CEST) Date: Fri, 22 May 2020 18:39:08 +0200 From: Peter Zijlstra To: "Ahmed S. Darwish" 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: <20200522163908.GP325280@hirez.programming.kicks-ass.net> References: <20200519214547.352050-1-a.darwish@linutronix.de> <20200519214547.352050-5-a.darwish@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200519214547.352050-5-a.darwish@linutronix.de> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.