From patchwork Thu Jul 1 11:35:37 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Hou Tao X-Patchwork-Id: 12353999 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,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 85B1DC11F64 for ; Thu, 1 Jul 2021 11:24:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6106B61467 for ; Thu, 1 Jul 2021 11:24:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236175AbhGAL1O (ORCPT ); Thu, 1 Jul 2021 07:27:14 -0400 Received: from szxga01-in.huawei.com ([45.249.212.187]:13048 "EHLO szxga01-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236040AbhGAL1N (ORCPT ); Thu, 1 Jul 2021 07:27:13 -0400 Received: from dggemv711-chm.china.huawei.com (unknown [172.30.72.53]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4GFwlK1mLBzZlx7; Thu, 1 Jul 2021 19:21:33 +0800 (CST) Received: from dggpeml500025.china.huawei.com (7.185.36.35) by dggemv711-chm.china.huawei.com (10.1.198.66) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Thu, 1 Jul 2021 19:24:40 +0800 Received: from huawei.com (10.175.124.27) by dggpeml500025.china.huawei.com (7.185.36.35) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Thu, 1 Jul 2021 19:24:39 +0800 From: Hou Tao To: Jens Axboe , Christoph Hellwig , "Alexander Viro" CC: , , , Subject: [PATCH] block: ensure the memory order between bi_private and bi_status Date: Thu, 1 Jul 2021 19:35:37 +0800 Message-ID: <20210701113537.582120-1-houtao1@huawei.com> X-Mailer: git-send-email 2.29.2 MIME-Version: 1.0 X-Originating-IP: [10.175.124.27] X-ClientProxiedBy: dggems703-chm.china.huawei.com (10.3.19.180) To dggpeml500025.china.huawei.com (7.185.36.35) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org When running stress test on null_blk under linux-4.19.y, the following warning is reported: percpu_ref_switch_to_atomic_rcu: percpu ref (css_release) <= 0 (-3) after switching to atomic The cause is that css_put() is invoked twice on the same bio as shown below: CPU 1: CPU 2: // IO completion kworker // IO submit thread __blkdev_direct_IO_simple submit_bio bio_endio bio_uninit(bio) css_put(bi_css) bi_css = NULL set_current_state(TASK_UNINTERRUPTIBLE) bio->bi_end_io blkdev_bio_end_io_simple bio->bi_private = NULL // bi_private is NULL READ_ONCE(bio->bi_private) wake_up_process smp_mb__after_spinlock bio_unint(bio) // read bi_css as no-NULL // so call css_put() again css_put(bi_css) Because there is no memory barriers between the reading and the writing of bi_private and bi_css, so reading bi_private as NULL can not guarantee bi_css will also be NULL on weak-memory model host (e.g, ARM64). For the latest kernel source, css_put() has been removed from bio_unint(), but the memory-order problem still exists, because the order between bio->bi_private and {bi_status|bi_blkg} is also assumed in __blkdev_direct_IO_simple(). It is reproducible that __blkdev_direct_IO_simple() may read bi_status as 0 event if bi_status is set as an errno in req_bio_endio(). In __blkdev_direct_IO(), the memory order between dio->waiter and dio->bio.bi_status is not guaranteed neither. Until now it is unable to reproduce it, maybe because dio->waiter and dio->bio.bi_status are in the same cache-line. But it is better to add guarantee for memory order. Fixing it by using smp_load_acquire() & smp_store_release() to guarantee the order between {bio->bi_private|dio->waiter} and {bi_status|bi_blkg}. Fixes: 189ce2b9dcc3 ("block: fast-path for small and simple direct I/O requests") Signed-off-by: Hou Tao --- fs/block_dev.c | 19 +++++++++++++++---- 1 file changed, 15 insertions(+), 4 deletions(-) diff --git a/fs/block_dev.c b/fs/block_dev.c index eb34f5c357cf..a602c6315b0b 100644 --- a/fs/block_dev.c +++ b/fs/block_dev.c @@ -224,7 +224,11 @@ static void blkdev_bio_end_io_simple(struct bio *bio) { struct task_struct *waiter = bio->bi_private; - WRITE_ONCE(bio->bi_private, NULL); + /* + * Paired with smp_load_acquire in __blkdev_direct_IO_simple() + * to ensure the order between bi_private and bi_xxx + */ + smp_store_release(&bio->bi_private, NULL); blk_wake_io_task(waiter); } @@ -283,7 +287,8 @@ __blkdev_direct_IO_simple(struct kiocb *iocb, struct iov_iter *iter, qc = submit_bio(&bio); for (;;) { set_current_state(TASK_UNINTERRUPTIBLE); - if (!READ_ONCE(bio.bi_private)) + /* Refer to comments in blkdev_bio_end_io_simple() */ + if (!smp_load_acquire(&bio.bi_private)) break; if (!(iocb->ki_flags & IOCB_HIPRI) || !blk_poll(bdev_get_queue(bdev), qc, true)) @@ -353,7 +358,12 @@ static void blkdev_bio_end_io(struct bio *bio) } else { struct task_struct *waiter = dio->waiter; - WRITE_ONCE(dio->waiter, NULL); + /* + * Paired with smp_load_acquire() in + * __blkdev_direct_IO() to ensure the order between + * dio->waiter and bio->bi_xxx + */ + smp_store_release(&dio->waiter, NULL); blk_wake_io_task(waiter); } } @@ -478,7 +488,8 @@ static ssize_t __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, for (;;) { set_current_state(TASK_UNINTERRUPTIBLE); - if (!READ_ONCE(dio->waiter)) + /* Refer to comments in blkdev_bio_end_io */ + if (!smp_load_acquire(&dio->waiter)) break; if (!(iocb->ki_flags & IOCB_HIPRI) ||