From patchwork Thu Mar 7 18:08:32 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Andrea Righi X-Patchwork-Id: 10843661 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 6D751139A for ; Thu, 7 Mar 2019 18:09:35 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 592BE2F46B for ; Thu, 7 Mar 2019 18:09:35 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 4B9F82F46D; Thu, 7 Mar 2019 18:09:35 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.9 required=2.0 tests=BAYES_00,MAILING_LIST_MULTI, RCVD_IN_DNSWL_NONE autolearn=ham version=3.3.1 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 599B72F46B for ; Thu, 7 Mar 2019 18:09:34 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 531A78E0004; Thu, 7 Mar 2019 13:09:32 -0500 (EST) Delivered-To: linux-mm-outgoing@kvack.org Received: by kanga.kvack.org (Postfix, from userid 40) id 41D238E0002; Thu, 7 Mar 2019 13:09:32 -0500 (EST) X-Original-To: int-list-linux-mm@kvack.org X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 2E5228E0004; Thu, 7 Mar 2019 13:09:32 -0500 (EST) X-Original-To: linux-mm@kvack.org X-Delivered-To: linux-mm@kvack.org Received: from mail-wr1-f71.google.com (mail-wr1-f71.google.com [209.85.221.71]) by kanga.kvack.org (Postfix) with ESMTP id CD54E8E0002 for ; Thu, 7 Mar 2019 13:09:31 -0500 (EST) Received: by mail-wr1-f71.google.com with SMTP id n2so1017804wrs.15 for ; Thu, 07 Mar 2019 10:09:31 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-original-authentication-results:x-gm-message-state:from:to:cc :subject:date:message-id:in-reply-to:references:mime-version :content-transfer-encoding; bh=E1LOBPaoUcwq7EBh5jwmgNQUJ5Ql4uc2em7Nrd4127w=; b=aj1Hkfw1NneMbh4OMd+ea7yqYhv3AD2aGetW5xpjy1qZ5fSvOwOGU0plClc3gtpa3m 3eBZCHc7o6G5Z3jTQQ8yVwt0lAkBmDzBL+gnqKp+YJbKPlsYyZkxhayHRlp1DQ5ZsueT 7zOU9DZauUWHj/jvB3eeEnTM0e31RUctckLFRsN92BHehHd4j4ihT1L0fwMDIendFuSC Gems4nIc7xFUTFx5e6d0IejD3ZnIs5nIAE7zk8AxniALAZeR0z60/hYmlCNz/TSFk99R XFXxcZpKxpv9g1o3Z28Izz7zUZQdiT8BVNAP4lxiec6pXOk1DWqVktfdsjJStH5AEKUy qcqA== X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of andrea.righi@canonical.com designates 91.189.89.112 as permitted sender) smtp.mailfrom=andrea.righi@canonical.com; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=canonical.com X-Gm-Message-State: APjAAAXDmHdlXaiCAVRvkdnZFrzHvV3LceQHCdIYBd7u4S7e6/Xtr+IP fY3iwvYPigbdvl2f32Io3Dgd/atyq4n214CFL6AAB4MXRySSqy4fLq4Rug/MqtgreAu/S8HHvM1 cCtPOHPKqvCPKlTPTYUIxma30Z4uUMGO/LRkpWiqK3faBhEnZdm2abZX/sBYs+ZTFoFdBcsGmQA 6jnxSUpV8q4L7H2rTfH+Z+4Tc0jnk56xLR9kJ725mOQLC9hnKWWSG8GQiFalIWPTr2gAj5UghOd STbw9rmFP5hB/+iJMmFDCcn2xqJyfiN6Zm+l4d0ASs9ZHJLQwxG3sdEEjN0kBklcGXu8JKYC6aU 15y7+3AZDbu6Qgx79c8iEFGm7SSKgrueW0Mh0tsiEnkbRLVzJqWOe4lKS1hKlU0NJrUrHtD3zgc Lp+eUfpQU3HN+jpaHz1rUZgBfri9ncyOWCjE1QpMQBfwUWeBnDxLYDwqsFyqEetjborj7BBbW5t zQGG8rZyKgRdbfZB6vJ+cg+cMPYsl7J1A0ptq9GVw+bB5mJZI6mv9sQxFhNp3Uru0JIvU49n6l6 zhxBfTlqUZeTX3WjmbpegUdL94ccTlNbgRHtVGo2OEB8rF+aEbh8PwjQdYghKUyCnHBziu6iKOi WKGdT1GL+AEm X-Received: by 2002:a5d:4090:: with SMTP id o16mr7646368wrp.208.1551982171122; Thu, 07 Mar 2019 10:09:31 -0800 (PST) X-Google-Smtp-Source: APXvYqyqzNbWOJoYqQpNNB3b39OmjVJX8fMudv/wdfPkvpD06Oq/B3GvtJqmAv6TdCgsJZXAk7+f X-Received: by 2002:a5d:4090:: with SMTP id o16mr7646288wrp.208.1551982169686; Thu, 07 Mar 2019 10:09:29 -0800 (PST) ARC-Seal: i=1; a=rsa-sha256; t=1551982169; cv=none; d=google.com; s=arc-20160816; b=wqKxUg01PFsU/JkDo2Jb6R/y1DbjYdemfyA9gzX3zfYb5/v+9XtaonBBTcXfawSVvw wNL1fqdJHA8pBjoIqTH7p9vNAP+cDMW7sqrEcTwFn3CzrNm9sOwcgnWQXFkkVMnrknzi 6FsXUmDYkNo9ij4bhRMViXgl467GtUtYEjFRDgX+ghnitgovkg33WkDy96Fi6xwu7sGA T5ZEmtGcmFhrrg0M60lr1phcytCnM7gnOvZ55lnwNwcuDfjTq8JMcsMvrnaEXmSTLZy9 hnpYHgYUcQrAzDAtM5E3aFEl//x/y6a9hT1WlYeLObKaJN7mAEe2xQ31nWsiJ53X2bvr rS+Q== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from; bh=E1LOBPaoUcwq7EBh5jwmgNQUJ5Ql4uc2em7Nrd4127w=; b=RHDgh0ET5iXFjVc/h4CvjYIHl3r0NBBV7RUzqKBHdFhtFg7O7TPulKwMbvqgvPL9BQ vw9fdWgo4yiI3smmeWMBU6pyhyM8Im/aN3ljTp4ZGnP45KoNDd6qMJf6TWtOTSPR8rTI JiFqJcxn7G6GZ+/f2Cvdxs4aZdmVTkev6HeqMbkDMmfrtIsZ6p3CQ8dYx+atQJo/J/i9 l9B400F73snYSvUM5CJdeui/T8fOoM4JMwl16jzY45Tsx7XYaiWQLEFbDz2AOyg5dPmI SY6NaIJw30iB+MlLB645i0g5BgjNOvrVoZwKtLrv1Egwhs0IjlnNKNewkoClVImReEOJ uAEQ== ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: best guess record for domain of andrea.righi@canonical.com designates 91.189.89.112 as permitted sender) smtp.mailfrom=andrea.righi@canonical.com; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=canonical.com Received: from youngberry.canonical.com (youngberry.canonical.com. [91.189.89.112]) by mx.google.com with ESMTPS id x9si3413134wru.443.2019.03.07.10.09.29 for (version=TLS1 cipher=AES128-SHA bits=128/128); Thu, 07 Mar 2019 10:09:29 -0800 (PST) Received-SPF: pass (google.com: best guess record for domain of andrea.righi@canonical.com designates 91.189.89.112 as permitted sender) client-ip=91.189.89.112; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of andrea.righi@canonical.com designates 91.189.89.112 as permitted sender) smtp.mailfrom=andrea.righi@canonical.com; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=canonical.com Received: from mail-wr1-f71.google.com ([209.85.221.71]) by youngberry.canonical.com with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.76) (envelope-from ) id 1h1xSS-0001kE-Rh for linux-mm@kvack.org; Thu, 07 Mar 2019 18:09:28 +0000 Received: by mail-wr1-f71.google.com with SMTP id b9so8962701wrw.14 for ; Thu, 07 Mar 2019 10:09:28 -0800 (PST) X-Received: by 2002:a1c:df07:: with SMTP id w7mr6623160wmg.23.1551982168470; Thu, 07 Mar 2019 10:09:28 -0800 (PST) X-Received: by 2002:a1c:df07:: with SMTP id w7mr6623133wmg.23.1551982168042; Thu, 07 Mar 2019 10:09:28 -0800 (PST) Received: from localhost.localdomain (host22-124-dynamic.46-79-r.retail.telecomitalia.it. [79.46.124.22]) by smtp.gmail.com with ESMTPSA id a74sm7872747wma.22.2019.03.07.10.09.26 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 07 Mar 2019 10:09:27 -0800 (PST) From: Andrea Righi To: Josef Bacik , Tejun Heo Cc: Li Zefan , Paolo Valente , Johannes Weiner , Jens Axboe , Vivek Goyal , Dennis Zhou , cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [PATCH v2 1/3] blkcg: prevent priority inversion problem during sync() Date: Thu, 7 Mar 2019 19:08:32 +0100 Message-Id: <20190307180834.22008-2-andrea.righi@canonical.com> X-Mailer: git-send-email 2.19.1 In-Reply-To: <20190307180834.22008-1-andrea.righi@canonical.com> References: <20190307180834.22008-1-andrea.righi@canonical.com> MIME-Version: 1.0 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: X-Virus-Scanned: ClamAV using ClamSMTP Prevent priority inversion problem when a high-priority blkcg issues a sync() and it is forced to wait the completion of all the writeback I/O generated by any other low-priority blkcg, causing massive latencies to processes that shouldn't be I/O-throttled at all. The idea is to save a list of blkcg's that are waiting for writeback: every time a sync() is executed the current blkcg is added to the list. Then, when I/O is throttled, if there's a blkcg waiting for writeback different than the current blkcg, no throttling is applied (we can probably refine this logic later, i.e., a better policy could be to adjust the throttling I/O rate using the blkcg with the highest speed from the list of waiters - priority inheritance, kinda). Signed-off-by: Andrea Righi --- block/blk-cgroup.c | 131 +++++++++++++++++++++++++++++++ block/blk-throttle.c | 11 ++- fs/fs-writeback.c | 5 ++ fs/sync.c | 8 +- include/linux/backing-dev-defs.h | 2 + include/linux/blk-cgroup.h | 23 ++++++ mm/backing-dev.c | 2 + 7 files changed, 178 insertions(+), 4 deletions(-) diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c index 2bed5725aa03..4305e78d1bb2 100644 --- a/block/blk-cgroup.c +++ b/block/blk-cgroup.c @@ -1351,6 +1351,137 @@ struct cgroup_subsys io_cgrp_subsys = { }; EXPORT_SYMBOL_GPL(io_cgrp_subsys); +#ifdef CONFIG_CGROUP_WRITEBACK +struct blkcg_wb_sleeper { + struct backing_dev_info *bdi; + struct blkcg *blkcg; + refcount_t refcnt; + struct list_head node; +}; + +static DEFINE_SPINLOCK(blkcg_wb_sleeper_lock); +static LIST_HEAD(blkcg_wb_sleeper_list); + +static struct blkcg_wb_sleeper * +blkcg_wb_sleeper_find(struct blkcg *blkcg, struct backing_dev_info *bdi) +{ + struct blkcg_wb_sleeper *bws; + + list_for_each_entry(bws, &blkcg_wb_sleeper_list, node) + if (bws->blkcg == blkcg && bws->bdi == bdi) + return bws; + return NULL; +} + +static void blkcg_wb_sleeper_add(struct blkcg_wb_sleeper *bws) +{ + list_add(&bws->node, &blkcg_wb_sleeper_list); +} + +static void blkcg_wb_sleeper_del(struct blkcg_wb_sleeper *bws) +{ + list_del_init(&bws->node); +} + +/** + * blkcg_wb_waiters_on_bdi - check for writeback waiters on a block device + * @blkcg: current blkcg cgroup + * @bdi: block device to check + * + * Return true if any other blkcg different than the current one is waiting for + * writeback on the target block device, false otherwise. + */ +bool blkcg_wb_waiters_on_bdi(struct blkcg *blkcg, struct backing_dev_info *bdi) +{ + struct blkcg_wb_sleeper *bws; + bool ret = false; + + spin_lock(&blkcg_wb_sleeper_lock); + list_for_each_entry(bws, &blkcg_wb_sleeper_list, node) + if (bws->bdi == bdi && bws->blkcg != blkcg) { + ret = true; + break; + } + spin_unlock(&blkcg_wb_sleeper_lock); + + return ret; +} + +/** + * blkcg_start_wb_wait_on_bdi - add current blkcg to writeback waiters list + * @bdi: target block device + * + * Add current blkcg to the list of writeback waiters on target block device. + */ +void blkcg_start_wb_wait_on_bdi(struct backing_dev_info *bdi) +{ + struct blkcg_wb_sleeper *new_bws, *bws; + struct blkcg *blkcg; + + new_bws = kzalloc(sizeof(*new_bws), GFP_KERNEL); + if (unlikely(!new_bws)) + return; + + rcu_read_lock(); + blkcg = blkcg_from_current(); + if (likely(blkcg)) { + /* Check if blkcg is already sleeping on bdi */ + spin_lock(&blkcg_wb_sleeper_lock); + bws = blkcg_wb_sleeper_find(blkcg, bdi); + if (bws) { + refcount_inc(&bws->refcnt); + } else { + /* Add current blkcg as a new wb sleeper on bdi */ + css_get(&blkcg->css); + new_bws->blkcg = blkcg; + new_bws->bdi = bdi; + refcount_set(&new_bws->refcnt, 1); + blkcg_wb_sleeper_add(new_bws); + new_bws = NULL; + } + spin_unlock(&blkcg_wb_sleeper_lock); + } + rcu_read_unlock(); + + kfree(new_bws); +} + +/** + * blkcg_stop_wb_wait_on_bdi - remove current blkcg from writeback waiters list + * @bdi: target block device + * + * Remove current blkcg from the list of writeback waiters on target block + * device. + */ +void blkcg_stop_wb_wait_on_bdi(struct backing_dev_info *bdi) +{ + struct blkcg_wb_sleeper *bws = NULL; + struct blkcg *blkcg; + + rcu_read_lock(); + blkcg = blkcg_from_current(); + if (!blkcg) { + rcu_read_unlock(); + return; + } + spin_lock(&blkcg_wb_sleeper_lock); + bws = blkcg_wb_sleeper_find(blkcg, bdi); + if (unlikely(!bws)) { + /* blkcg_start/stop_wb_wait_on_bdi() mismatch */ + WARN_ON(1); + goto out_unlock; + } + if (refcount_dec_and_test(&bws->refcnt)) { + blkcg_wb_sleeper_del(bws); + css_put(&blkcg->css); + kfree(bws); + } +out_unlock: + spin_unlock(&blkcg_wb_sleeper_lock); + rcu_read_unlock(); +} +#endif + /** * blkcg_activate_policy - activate a blkcg policy on a request_queue * @q: request_queue of interest diff --git a/block/blk-throttle.c b/block/blk-throttle.c index 1b97a73d2fb1..da817896cded 100644 --- a/block/blk-throttle.c +++ b/block/blk-throttle.c @@ -970,9 +970,13 @@ static bool tg_may_dispatch(struct throtl_grp *tg, struct bio *bio, { bool rw = bio_data_dir(bio); unsigned long bps_wait = 0, iops_wait = 0, max_wait = 0; + struct throtl_data *td = tg->td; + struct request_queue *q = td->queue; + struct backing_dev_info *bdi = q->backing_dev_info; + struct blkcg_gq *blkg = tg_to_blkg(tg); /* - * Currently whole state machine of group depends on first bio + * Currently whole state machine of group depends on first bio * queued in the group bio list. So one should not be calling * this function with a different bio if there are other bios * queued. @@ -981,8 +985,9 @@ static bool tg_may_dispatch(struct throtl_grp *tg, struct bio *bio, bio != throtl_peek_queued(&tg->service_queue.queued[rw])); /* If tg->bps = -1, then BW is unlimited */ - if (tg_bps_limit(tg, rw) == U64_MAX && - tg_iops_limit(tg, rw) == UINT_MAX) { + if (blkcg_wb_waiters_on_bdi(blkg->blkcg, bdi) || + (tg_bps_limit(tg, rw) == U64_MAX && + tg_iops_limit(tg, rw) == UINT_MAX)) { if (wait) *wait = 0; return true; diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index 36855c1f8daf..77c039a0ec25 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -28,6 +28,7 @@ #include #include #include +#include #include "internal.h" /* @@ -2446,6 +2447,8 @@ void sync_inodes_sb(struct super_block *sb) return; WARN_ON(!rwsem_is_locked(&sb->s_umount)); + blkcg_start_wb_wait_on_bdi(bdi); + /* protect against inode wb switch, see inode_switch_wbs_work_fn() */ bdi_down_write_wb_switch_rwsem(bdi); bdi_split_work_to_wbs(bdi, &work, false); @@ -2453,6 +2456,8 @@ void sync_inodes_sb(struct super_block *sb) bdi_up_write_wb_switch_rwsem(bdi); wait_sb_inodes(sb); + + blkcg_stop_wb_wait_on_bdi(bdi); } EXPORT_SYMBOL(sync_inodes_sb); diff --git a/fs/sync.c b/fs/sync.c index b54e0541ad89..3958b8f98b85 100644 --- a/fs/sync.c +++ b/fs/sync.c @@ -16,6 +16,7 @@ #include #include #include +#include #include "internal.h" #define VALID_FLAGS (SYNC_FILE_RANGE_WAIT_BEFORE|SYNC_FILE_RANGE_WRITE| \ @@ -76,8 +77,13 @@ static void sync_inodes_one_sb(struct super_block *sb, void *arg) static void sync_fs_one_sb(struct super_block *sb, void *arg) { - if (!sb_rdonly(sb) && sb->s_op->sync_fs) + struct backing_dev_info *bdi = sb->s_bdi; + + if (!sb_rdonly(sb) && sb->s_op->sync_fs) { + blkcg_start_wb_wait_on_bdi(bdi); sb->s_op->sync_fs(sb, *(int *)arg); + blkcg_stop_wb_wait_on_bdi(bdi); + } } static void fdatawrite_one_bdev(struct block_device *bdev, void *arg) diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h index 07e02d6df5ad..095e4dd0427b 100644 --- a/include/linux/backing-dev-defs.h +++ b/include/linux/backing-dev-defs.h @@ -191,6 +191,8 @@ struct backing_dev_info { struct rb_root cgwb_congested_tree; /* their congested states */ struct mutex cgwb_release_mutex; /* protect shutdown of wb structs */ struct rw_semaphore wb_switch_rwsem; /* no cgwb switch while syncing */ + struct list_head cgwb_waiters; /* list of all waiters for writeback */ + spinlock_t cgwb_waiters_lock; /* protect cgwb_waiters list */ #else struct bdi_writeback_congested *wb_congested; #endif diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h index 76c61318fda5..0f7dcb70e922 100644 --- a/include/linux/blk-cgroup.h +++ b/include/linux/blk-cgroup.h @@ -56,6 +56,7 @@ struct blkcg { struct list_head all_blkcgs_node; #ifdef CONFIG_CGROUP_WRITEBACK + struct list_head cgwb_wait_node; struct list_head cgwb_list; refcount_t cgwb_refcnt; #endif @@ -252,6 +253,12 @@ static inline struct blkcg *css_to_blkcg(struct cgroup_subsys_state *css) return css ? container_of(css, struct blkcg, css) : NULL; } +static inline struct blkcg *blkcg_from_current(void) +{ + WARN_ON_ONCE(!rcu_read_lock_held()); + return css_to_blkcg(blkcg_css()); +} + /** * __bio_blkcg - internal, inconsistent version to get blkcg * @@ -454,6 +461,10 @@ static inline void blkcg_cgwb_put(struct blkcg *blkcg) blkcg_destroy_blkgs(blkcg); } +bool blkcg_wb_waiters_on_bdi(struct blkcg *blkcg, struct backing_dev_info *bdi); +void blkcg_start_wb_wait_on_bdi(struct backing_dev_info *bdi); +void blkcg_stop_wb_wait_on_bdi(struct backing_dev_info *bdi); + #else static inline void blkcg_cgwb_get(struct blkcg *blkcg) { } @@ -464,6 +475,14 @@ static inline void blkcg_cgwb_put(struct blkcg *blkcg) blkcg_destroy_blkgs(blkcg); } +static inline bool +blkcg_wb_waiters_on_bdi(struct blkcg *blkcg, struct backing_dev_info *bdi) +{ + return false; +} +static inline void blkcg_start_wb_wait_on_bdi(struct backing_dev_info *bdi) { } +static inline void blkcg_stop_wb_wait_on_bdi(struct backing_dev_info *bdi) { } + #endif /** @@ -772,6 +791,7 @@ static inline void blkcg_bio_issue_init(struct bio *bio) static inline bool blkcg_bio_issue_check(struct request_queue *q, struct bio *bio) { + struct backing_dev_info *bdi = q->backing_dev_info; struct blkcg_gq *blkg; bool throtl = false; @@ -788,6 +808,9 @@ static inline bool blkcg_bio_issue_check(struct request_queue *q, blkg = bio->bi_blkg; + if (blkcg_wb_waiters_on_bdi(blkg->blkcg, bdi)) + bio_set_flag(bio, BIO_THROTTLED); + throtl = blk_throtl_bio(q, blkg, bio); if (!throtl) { diff --git a/mm/backing-dev.c b/mm/backing-dev.c index 72e6d0c55cfa..8848d26e8bf6 100644 --- a/mm/backing-dev.c +++ b/mm/backing-dev.c @@ -686,10 +686,12 @@ static int cgwb_bdi_init(struct backing_dev_info *bdi) { int ret; + INIT_LIST_HEAD(&bdi->cgwb_waiters); INIT_RADIX_TREE(&bdi->cgwb_tree, GFP_ATOMIC); bdi->cgwb_congested_tree = RB_ROOT; mutex_init(&bdi->cgwb_release_mutex); init_rwsem(&bdi->wb_switch_rwsem); + spin_lock_init(&bdi->cgwb_waiters_lock); ret = wb_init(&bdi->wb, bdi, 1, GFP_KERNEL); if (!ret) {