From patchwork Tue Mar 24 21:48:27 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Doug Anderson X-Patchwork-Id: 11456619 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id ACCD81667 for ; Tue, 24 Mar 2020 21:48:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8F5262077D for ; Tue, 24 Mar 2020 21:48:48 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b="AnRH8geS" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728291AbgCXVss (ORCPT ); Tue, 24 Mar 2020 17:48:48 -0400 Received: from mail-pg1-f194.google.com ([209.85.215.194]:44585 "EHLO mail-pg1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728094AbgCXVsr (ORCPT ); Tue, 24 Mar 2020 17:48:47 -0400 Received: by mail-pg1-f194.google.com with SMTP id 142so93079pgf.11 for ; Tue, 24 Mar 2020 14:48:46 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=DsvR+TcwmI4jVJzuydsxBMDLVq1XTHOSTRk3ilTd2M8=; b=AnRH8geSYlcU2enKayiCXSAWBghoMs55YsWkfBp+R9HE3GPrNP9BFUgaw3qS3gdH+K ZkSGwpXzzU20JbA/DvnYdeqevg9YGr2QofJvky1HTIStAjLmX+Cc7NhUgYoUAooveuG2 sgr+DfmyNURrvjjgktbXgASyYM7ekCHEJJxNk= 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:mime-version :content-transfer-encoding; bh=DsvR+TcwmI4jVJzuydsxBMDLVq1XTHOSTRk3ilTd2M8=; b=tcBFPhRCs7negw5IuH1veA85naRZhzHfdZqkgIJBJWttodnsLTc68DO57GmUlm1HmW YDo/jRYNQQlHMMRmT7A5zp7DiCLVZxYnPg3oOfH+1aePmGUonJAwtrfn8YHPe07z9drN GyufqW11Vz++W8FscVpVJQD9w+pSwnFgLjhqkc3uvT2TARs+jaeqWISzzNbFsIfbze3u NyOBz2W7mIiQbLSEidQ6gonR73YF9Nw/yW/OurRFIFD6njnH6GrLrWfPfpOyZvaga1vc wV/zdE8DgyOqoUhNM9iQ1f5Q5i6d68pOVLINChNilCPUc4lwD94Gmbo+MT4/4GDJkpOD 4oXA== X-Gm-Message-State: ANhLgQ1mK4lhVxpLOMpxX1spxR1bpJhGDbxGcAqXOjpn0qbAc6DLcjDR SQTMkBKvRxl6PegQvXqU2IuO6w== X-Google-Smtp-Source: ADFU+vtQ3zmuuk6S7MV6yqPmtIItc12Pcv44PxtKGobUKPTjqQaUljsbCnX/DNGWIEQnxG1Z5CiMWw== X-Received: by 2002:aa7:999e:: with SMTP id k30mr30994163pfh.235.1585086526229; Tue, 24 Mar 2020 14:48:46 -0700 (PDT) Received: from tictac2.mtv.corp.google.com ([2620:15c:202:1:24fa:e766:52c9:e3b2]) by smtp.gmail.com with ESMTPSA id l62sm15578328pgd.82.2020.03.24.14.48.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 24 Mar 2020 14:48:45 -0700 (PDT) From: Douglas Anderson To: Alexander Viro Cc: Salman Qazi , Guenter Roeck , Paolo Valente , Christoph Hellwig , Douglas Anderson , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v2] bdev: Reduce time holding bd_mutex in sync in blkdev_close() Date: Tue, 24 Mar 2020 14:48:27 -0700 Message-Id: <20200324144754.v2.1.I9df0264e151a740be292ad3ee3825f31b5997776@changeid> X-Mailer: git-send-email 2.25.1.696.g5e7596f4ac-goog MIME-Version: 1.0 Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org While trying to "dd" to the block device for a USB stick, I encountered a hung task warning (blocked for > 120 seconds). I managed to come up with an easy way to reproduce this on my system (where /dev/sdb is the block device for my USB stick) with: while true; do dd if=/dev/zero of=/dev/sdb bs=4M; done With my reproduction here are the relevant bits from the hung task detector: INFO: task udevd:294 blocked for more than 122 seconds. ... udevd D 0 294 1 0x00400008 Call trace: ... mutex_lock_nested+0x40/0x50 __blkdev_get+0x7c/0x3d4 blkdev_get+0x118/0x138 blkdev_open+0x94/0xa8 do_dentry_open+0x268/0x3a0 vfs_open+0x34/0x40 path_openat+0x39c/0xdf4 do_filp_open+0x90/0x10c do_sys_open+0x150/0x3c8 ... ... Showing all locks held in the system: ... 1 lock held by dd/2798: #0: ffffff814ac1a3b8 (&bdev->bd_mutex){+.+.}, at: __blkdev_put+0x50/0x204 ... dd D 0 2798 2764 0x00400208 Call trace: ... schedule+0x8c/0xbc io_schedule+0x1c/0x40 wait_on_page_bit_common+0x238/0x338 __lock_page+0x5c/0x68 write_cache_pages+0x194/0x500 generic_writepages+0x64/0xa4 blkdev_writepages+0x24/0x30 do_writepages+0x48/0xa8 __filemap_fdatawrite_range+0xac/0xd8 filemap_write_and_wait+0x30/0x84 __blkdev_put+0x88/0x204 blkdev_put+0xc4/0xe4 blkdev_close+0x28/0x38 __fput+0xe0/0x238 ____fput+0x1c/0x28 task_work_run+0xb0/0xe4 do_notify_resume+0xfc0/0x14bc work_pending+0x8/0x14 The problem appears related to the fact that my USB disk is terribly slow and that I have a lot of RAM in my system to cache things. Specifically my writes seem to be happening at ~15 MB/s and I've got ~4 GB of RAM in my system that can be used for buffering. To write 4 GB of buffer to disk thus takes ~4000 MB / ~15 MB/s = ~267 seconds. The 267 second number is a problem because in __blkdev_put() we call sync_blockdev() while holding the bd_mutex. Any other callers who want the bd_mutex will be blocked for the whole time. The problem is made worse because I believe blkdev_put() specifically tells other tasks (namely udev) to go try to access the device at right around the same time we're going to hold the mutex for a long time. Putting some traces around this (after disabling the hung task detector), I could confirm: dd: 437.608600: __blkdev_put() right before sync_blockdev() for sdb udevd: 437.623901: blkdev_open() right before blkdev_get() for sdb dd: 661.468451: __blkdev_put() right after sync_blockdev() for sdb udevd: 663.820426: blkdev_open() right after blkdev_get() for sdb A simple fix for this is to realize that sync_blockdev() works fine if you're not holding the mutex. Also, it's not the end of the world if you sync a little early (though it can have performance impacts). Thus we can make a guess that we're going to need to do the sync and then do it without holding the mutex. We still do one last sync with the mutex but it should be much, much faster. With this, my hung task warnings for my test case are gone. Signed-off-by: Douglas Anderson Reviewed-by: Guenter Roeck Reviewed-by: Christoph Hellwig --- I didn't put a "Fixes" annotation here because, as far as I can tell, this issue has been here "forever" unless someone knows of something else that changed that made this possible to hit. This could probably get picked back to any stable tree that anyone is still maintaining. Changes in v2: - Don't bother holding the mutex when checking "bd_openers". fs/block_dev.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/fs/block_dev.c b/fs/block_dev.c index 9501880dff5e..40c57a9cc91a 100644 --- a/fs/block_dev.c +++ b/fs/block_dev.c @@ -1892,6 +1892,16 @@ static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part) struct gendisk *disk = bdev->bd_disk; struct block_device *victim = NULL; + /* + * Sync early if it looks like we're the last one. If someone else + * opens the block device between now and the decrement of bd_openers + * then we did a sync that we didn't need to, but that's not the end + * of the world and we want to avoid long (could be several minute) + * syncs while holding the mutex. + */ + if (bdev->bd_openers == 1) + sync_blockdev(bdev); + mutex_lock_nested(&bdev->bd_mutex, for_part); if (for_part) bdev->bd_part_count--;