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=-22.4 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_GIT,USER_IN_DEF_DKIM_WL autolearn=ham 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 31B84C5DF60 for ; Tue, 5 Nov 2019 18:28:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 06436214D8 for ; Tue, 5 Nov 2019 18:28:39 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="vPxN2a9e" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2390668AbfKES2i (ORCPT ); Tue, 5 Nov 2019 13:28:38 -0500 Received: from mail-qt1-f202.google.com ([209.85.160.202]:34056 "EHLO mail-qt1-f202.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389724AbfKES2i (ORCPT ); Tue, 5 Nov 2019 13:28:38 -0500 Received: by mail-qt1-f202.google.com with SMTP id u26so23331169qtq.1 for ; Tue, 05 Nov 2019 10:28:36 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:message-id:mime-version:subject:from:to:cc; bh=wgxjvwX6+cnB3A8RMM5lxB/OlF4KhSWsamiPqNj2U6w=; b=vPxN2a9eb5x/Kf9/TWrUXR40il0raZDiN6LYAMVyDiDfjpf7s9xXS6bjE5/awxsuoy 0q1m5Zb8UvHXS6AiO3q0DRqBZT21rMVwU80N064o5cR+O0LTltXWOTW6KWnDg6PzLmHb 9M1FDkcKNGd4va1a96WGR8KkuNh54njhXe7jlzCokkcBEA+WXz3ori13j//Kq+ltDpEN seY173tKGH4fj+4kOxr/zl/3VnAdEcGpqai8VOCJLmWbOUj4Y1MH2s/elBVlj92QhMKX CRGRDyk5VgA1XMd5XFM8hjSqA1lFfraXvc5TeTIXEEbBNjacO726/dEGWgneMJa9+qcG 1vGA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:message-id:mime-version:subject:from:to:cc; bh=wgxjvwX6+cnB3A8RMM5lxB/OlF4KhSWsamiPqNj2U6w=; b=ShSDxFh+gNjnWxmw0tXTZ9D1K9D4oqjzWuLcGKFUD6vpH457Qd9HnhN+OfEAX3GVGI OtVYSUDv+QaGs1cBw74oSF5aYlBG0dntQonlddAOjTJHF6oAd7I8yk3M9q4JdWJKYbfl RKgym0au+7Zr7TLSYBwHFoeYwEIMkS7qwqQ3Kgu6XtqTtyNiTRd08WETWExMxXeBCZeb S7/CuZbGrdXPIKOnnHmAt9tvNEV0XgArxPIMjW7apRsZl0nmWAPUu4VJZQR8a+mz7chp KeyFMbAhOODS0iQEbRCg3MQRh7+Uae3UNBWAl5p+JoectLRBMW2HCzy8F9Vu4uPcGeSN 5PrA== X-Gm-Message-State: APjAAAVALHTdIG9ivpJgrBImb/NJNKKOjUSCHUmpMjM0WBOAAMqjDrIp r6tirLvAE8j9Ah16aL2bgkk/pD67gg== X-Google-Smtp-Source: APXvYqzTpHCaI8bTtovq6RT1gOrzAFSaWFSHvNTuvzdVkIjREWwMwBmhqgfZhpxg2SpG0OZOyHCv1UTJ7g== X-Received: by 2002:a37:9d44:: with SMTP id g65mr27690527qke.302.1572978515493; Tue, 05 Nov 2019 10:28:35 -0800 (PST) Date: Tue, 5 Nov 2019 19:27:47 +0100 Message-Id: <20191105182746.217864-1-elver@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.24.0.rc1.363.gb1bccd3e3d-goog Subject: [PATCH] blk-wbt: Fix data race and avoid possible false sharing From: Marco Elver To: elver@google.com Cc: axboe@kernel.dk, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, syzbot+ba8947364367f96fe16b@syzkaller.appspotmail.com Content-Type: text/plain; charset="UTF-8" Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org The pattern here is to avoid possible false sharing. However, due to compiler optimizations the code may simply collapse to the write if we omit READ_ONCE/WRITE_ONCE: https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE#it-may-improve-performance ================================================================== BUG: KCSAN: data-race in wbt_wait / wbt_wait read to 0xffff88821aa6d140 of 8 bytes by task 10372 on cpu 1: wb_timestamp block/blk-wbt.c:88 [inline] wb_timestamp block/blk-wbt.c:83 [inline] wbt_wait+0x1f9/0x250 block/blk-wbt.c:587 __rq_qos_throttle+0x47/0x70 block/blk-rq-qos.c:72 rq_qos_throttle block/blk-rq-qos.h:185 [inline] blk_mq_make_request+0x29c/0xf60 block/blk-mq.c:1971 generic_make_request block/blk-core.c:1064 [inline] generic_make_request+0x196/0x740 block/blk-core.c:1006 submit_bio+0x96/0x3c0 block/blk-core.c:1190 submit_bh_wbc+0x40f/0x460 fs/buffer.c:3095 submit_bh fs/buffer.c:3101 [inline] __bread_slow fs/buffer.c:1177 [inline] __bread_gfp+0xe7/0x1e0 fs/buffer.c:1359 sb_bread include/linux/buffer_head.h:307 [inline] fat__get_entry+0x35e/0x4f0 fs/fat/dir.c:100 fat_get_entry fs/fat/dir.c:128 [inline] fat_get_short_entry+0x103/0x200 fs/fat/dir.c:877 fat_subdirs+0x6b/0x110 fs/fat/dir.c:943 fat_read_root fs/fat/inode.c:1416 [inline] fat_fill_super+0x1552/0x1f50 fs/fat/inode.c:1862 vfat_fill_super+0x3b/0x50 fs/fat/namei_vfat.c:1050 mount_bdev+0x262/0x2d0 fs/super.c:1415 vfat_mount+0x3e/0x60 fs/fat/namei_vfat.c:1057 write to 0xffff88821aa6d140 of 8 bytes by task 10375 on cpu 0: wb_timestamp block/blk-wbt.c:89 [inline] wb_timestamp block/blk-wbt.c:83 [inline] wbt_wait+0x21e/0x250 block/blk-wbt.c:587 __rq_qos_throttle+0x47/0x70 block/blk-rq-qos.c:72 rq_qos_throttle block/blk-rq-qos.h:185 [inline] blk_mq_make_request+0x29c/0xf60 block/blk-mq.c:1971 generic_make_request block/blk-core.c:1064 [inline] generic_make_request+0x196/0x740 block/blk-core.c:1006 submit_bio+0x96/0x3c0 block/blk-core.c:1190 mpage_bio_submit fs/mpage.c:66 [inline] mpage_readpages+0x36c/0x3c0 fs/mpage.c:410 blkdev_readpages+0x36/0x50 fs/block_dev.c:620 read_pages+0xa2/0x2d0 mm/readahead.c:126 __do_page_cache_readahead+0x353/0x390 mm/readahead.c:212 force_page_cache_readahead+0x13a/0x1f0 mm/readahead.c:243 page_cache_sync_readahead+0x1cf/0x1e0 mm/readahead.c:522 generic_file_buffered_read mm/filemap.c:2050 [inline] generic_file_read_iter+0xeb6/0x1440 mm/filemap.c:2323 blkdev_read_iter+0xb2/0xe0 fs/block_dev.c:2010 Reported by Kernel Concurrency Sanitizer on: CPU: 0 PID: 10375 Comm: blkid Not tainted 5.4.0-rc3+ #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 ================================================================== Reported-by: syzbot+ba8947364367f96fe16b@syzkaller.appspotmail.com Signed-off-by: Marco Elver --- block/blk-wbt.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/block/blk-wbt.c b/block/blk-wbt.c index 8641ba9793c5..ce281a9007a6 100644 --- a/block/blk-wbt.c +++ b/block/blk-wbt.c @@ -85,8 +85,8 @@ static void wb_timestamp(struct rq_wb *rwb, unsigned long *var) if (rwb_enabled(rwb)) { const unsigned long cur = jiffies; - if (cur != *var) - *var = cur; + if (cur != READ_ONCE(*var)) + WRITE_ONCE(*var, cur); } } -- 2.24.0.rc1.363.gb1bccd3e3d-goog