From patchwork Mon Jul 12 23:05:25 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Matteo Croce X-Patchwork-Id: 12372539 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=-16.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT 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 C0639C07E99 for ; Mon, 12 Jul 2021 23:07:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 989DC60FE7 for ; Mon, 12 Jul 2021 23:07:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230019AbhGLXKa (ORCPT ); Mon, 12 Jul 2021 19:10:30 -0400 Received: from mail-ed1-f51.google.com ([209.85.208.51]:46957 "EHLO mail-ed1-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229562AbhGLXKa (ORCPT ); Mon, 12 Jul 2021 19:10:30 -0400 Received: by mail-ed1-f51.google.com with SMTP id s15so30381070edt.13; Mon, 12 Jul 2021 16:07:39 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=5pjMesyq1pHdjXnthdtuE3bdQ3vGQE2JW/5oUX1BQXU=; b=YSVW5/xI8ofU59jvslGKgFWkYUgAm1kQPUlr1BISdsSrizDOAQlx/IB7njkbfvIbe+ CIZ+MnRG7NVHlEkBZXKCLSC37tyAud+8sTnEUbYErvKR7LKw2XZ4ulOLwxHcnzRgAE1S ULMgCYYzV6wZBYWnAajlLjtkdGQUj4KDtWqcPGO0AnFF8AMfrxdEjOArCZ0GfzCDQzdN pORaCHkAyYgsqUWhXcS5iYTiq+nQsZK7ZVA7neXAZz3EM0M0gFeJgRb8T1UBYiUZQIvk lbHHEcnsA3DEIDj3Wb9jdVVxGfMT9i3H3NiXkEsVvd0on5L9h+hEktLJL1668HdJNtka T/Yw== X-Gm-Message-State: AOAM530JGs4o7S1KQRwJV636WCyRv1QlQGA3yr0eWFfAfsIGvHdnWXzq Oh+1U8WqFU5AaPXgmpvgfgZ1rnGZHezhrg== X-Google-Smtp-Source: ABdhPJzRs8hZEOn5FeEYmqrhp/UfboiOSx2r9fFxULoGASfYLNNoR2jZYMJ2ABprpIWpuLAz7ImTIA== X-Received: by 2002:a05:6402:6:: with SMTP id d6mr1567340edu.236.1626131259232; Mon, 12 Jul 2021 16:07:39 -0700 (PDT) Received: from msft-t490s.fritz.box (host-95-250-115-52.retail.telecomitalia.it. [95.250.115.52]) by smtp.gmail.com with ESMTPSA id h3sm5494111ejf.53.2021.07.12.16.07.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 12 Jul 2021 16:07:38 -0700 (PDT) From: Matteo Croce To: linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, Jens Axboe , Christoph Hellwig Cc: linux-kernel@vger.kernel.org, Lennart Poettering , Luca Boccassi , Alexander Viro , Damien Le Moal , Tejun Heo , =?utf-8?q?Javier_Gonz=C3=A1lez?= , Niklas Cassel , Johannes Thumshirn , Hannes Reinecke , Matthew Wilcox , JeffleXu Subject: [PATCH v5 1/6] block: add disk sequence number Date: Tue, 13 Jul 2021 01:05:25 +0200 Message-Id: <20210712230530.29323-2-mcroce@linux.microsoft.com> X-Mailer: git-send-email 2.31.1 In-Reply-To: <20210712230530.29323-1-mcroce@linux.microsoft.com> References: <20210712230530.29323-1-mcroce@linux.microsoft.com> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org From: Matteo Croce Associating uevents with block devices in userspace is difficult and racy: the uevent netlink socket is lossy, and on slow and overloaded systems has a very high latency. Block devices do not have exclusive owners in userspace, any process can set one up (e.g. loop devices). Moreover, device names can be reused (e.g. loop0 can be reused again and again). A userspace process setting up a block device and watching for its events cannot thus reliably tell whether an event relates to the device it just set up or another earlier instance with the same name. Being able to set a UUID on a loop device would solve the race conditions. But it does not allow to derive orderings from uevents: if you see a uevent with a UUID that does not match the device you are waiting for, you cannot tell whether it's because the right uevent has not arrived yet, or it was already sent and you missed it. So you cannot tell whether you should wait for it or not. Associating a unique, monotonically increasing sequential number to the lifetime of each block device, which can be retrieved with an ioctl immediately upon setting it up, allows to solve the race conditions with uevents, and also allows userspace processes to know whether they should wait for the uevent they need or if it was dropped and thus they should move on. Additionally, increment the disk sequence number when the media change, i.e. on DISK_EVENT_MEDIA_CHANGE event. Reviewed-by: Christoph Hellwig Signed-off-by: Matteo Croce --- block/disk-events.c | 3 +++ block/genhd.c | 24 ++++++++++++++++++++++++ include/linux/genhd.h | 2 ++ 3 files changed, 29 insertions(+) diff --git a/block/disk-events.c b/block/disk-events.c index a75931ff5da4..04c52f3992ed 100644 --- a/block/disk-events.c +++ b/block/disk-events.c @@ -190,6 +190,9 @@ static void disk_check_events(struct disk_events *ev, spin_unlock_irq(&ev->lock); + if (events & DISK_EVENT_MEDIA_CHANGE) + inc_diskseq(disk); + /* * Tell userland about new events. Only the events listed in * @disk->events are reported, and only if DISK_EVENT_FLAG_UEVENT diff --git a/block/genhd.c b/block/genhd.c index af4d2ab4a633..0be32dbe97bb 100644 --- a/block/genhd.c +++ b/block/genhd.c @@ -29,6 +29,23 @@ static struct kobject *block_depr; +/* + * Unique, monotonically increasing sequential number associated with block + * devices instances (i.e. incremented each time a device is attached). + * Associating uevents with block devices in userspace is difficult and racy: + * the uevent netlink socket is lossy, and on slow and overloaded systems has + * a very high latency. + * Block devices do not have exclusive owners in userspace, any process can set + * one up (e.g. loop devices). Moreover, device names can be reused (e.g. loop0 + * can be reused again and again). + * A userspace process setting up a block device and watching for its events + * cannot thus reliably tell whether an event relates to the device it just set + * up or another earlier instance with the same name. + * This sequential number allows userspace processes to solve this problem, and + * uniquely associate an uevent to the lifetime to a device. + */ +static atomic64_t diskseq; + /* for extended dynamic devt allocation, currently only one major is used */ #define NR_EXT_DEVT (1 << MINORBITS) static DEFINE_IDA(ext_devt_ida); @@ -1263,6 +1280,8 @@ struct gendisk *__alloc_disk_node(int minors, int node_id) disk_to_dev(disk)->class = &block_class; disk_to_dev(disk)->type = &disk_type; device_initialize(disk_to_dev(disk)); + inc_diskseq(disk); + return disk; out_destroy_part_tbl: @@ -1363,3 +1382,8 @@ int bdev_read_only(struct block_device *bdev) return bdev->bd_read_only || get_disk_ro(bdev->bd_disk); } EXPORT_SYMBOL(bdev_read_only); + +void inc_diskseq(struct gendisk *disk) +{ + disk->diskseq = atomic64_inc_return(&diskseq); +} diff --git a/include/linux/genhd.h b/include/linux/genhd.h index 13b34177cc85..140c028845af 100644 --- a/include/linux/genhd.h +++ b/include/linux/genhd.h @@ -172,6 +172,7 @@ struct gendisk { int node_id; struct badblocks *bb; struct lockdep_map lockdep_map; + u64 diskseq; }; /* @@ -332,6 +333,7 @@ static inline void bd_unlink_disk_holder(struct block_device *bdev, #endif /* CONFIG_SYSFS */ dev_t part_devt(struct gendisk *disk, u8 partno); +void inc_diskseq(struct gendisk *disk); dev_t blk_lookup_devt(const char *name, int partno); void blk_request_module(dev_t devt); #ifdef CONFIG_BLOCK