From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-131.freemail.mail.aliyun.com (out30-131.freemail.mail.aliyun.com [115.124.30.131]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CBFF83BE17F; Sun, 17 May 2026 14:22:24 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.131 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779027757; cv=none; b=Q6BxTQFA64xeynhNdoIOclwY4ocKYgBM9wTZAKjK2s15SFDuflLxZJyxI1HJ9oAse5RuPakGZSVQEbPX1ir2QMEG46SEfJHYUdOpPJHGqyi7XBAnINhpI5Iq8GADQt5tVWy8e6s1ecuxzk6NZc7qDKyTsBMQK6s4aM3A1dQHG4w= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779027757; c=relaxed/simple; bh=iVqgbsmoUShKzc8sehabHkz/7kiOXcuiLF9YU7dEtZU=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=s7+dVxnsKXtUmxCOlszY3zmtTMc2E7IwbTAJ/VjB9zOebZPc3Pmj7LwdBPAtFfWf1j7O1MZ7Z04kuU0yfFGl+RV1nQAXyluQDRSr3TsK79PPg1zb+dgZRVotsD54V8DeKhNsUK0v8xx8mUmrT87NeBtWJd9ZENNMsY9i5fX/+ZM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=HoR/2PVr; arc=none smtp.client-ip=115.124.30.131 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="HoR/2PVr" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1779027732; h=From:To:Subject:Date:Message-ID:MIME-Version; bh=pEeH7zHkW8IEVUYlNw8sKefpVLWTLyEFqi5D7xTTfZ0=; b=HoR/2PVrdJE7Ku1lN6WOTGMpKWhHTKWqhVAT378NGf/d4VB/K/4p4N+7pJp4Ax+S4wNfZHqR4ovIc0yo0+vidrFMo/o1iDpcAFaYlpCDHToLRyAnIWG55zF8D882rVTa47lhUVgXfPcluOSfb5Y8I8haRstd6tJZxurj12nKP2U= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R111e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033045133197;MF=libaokun@linux.alibaba.com;NM=1;PH=DS;RN=8;SR=0;TI=SMTPD_---0X31pQEL_1779027731; Received: from x31h02109.sqa.na131.tbsite.net(mailfrom:libaokun@linux.alibaba.com fp:SMTPD_---0X31pQEL_1779027731 cluster:ay36) by smtp.aliyun-inc.com; Sun, 17 May 2026 22:22:12 +0800 From: Baokun Li To: linux-fsdevel@vger.kernel.org Cc: viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz, tj@kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v2 1/3] writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs() Date: Sun, 17 May 2026 22:21:30 +0800 Message-ID: <20260517142147.3354909-2-libaokun@linux.alibaba.com> X-Mailer: git-send-email 2.43.7 In-Reply-To: <20260517142147.3354909-1-libaokun@linux.alibaba.com> References: <20260517142147.3354909-1-libaokun@linux.alibaba.com> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit When a container exits, the following BUG_ON() is occasionally triggered: ================================================================== VFS: Busy inodes after unmount of sdb (ext4) ------------[ cut here ]------------ kernel BUG at fs/super.c:695! CPU: 3 PID: 6 Comm: containerd-shim Tainted: G OE K 6.6 #1 pstate: 63400009 (nZCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--) pc : generic_shutdown_super+0xf0/0x100 lr : generic_shutdown_super+0xf0/0x100 Call trace: generic_shutdown_super+0xf0/0x100 kill_block_super+0x20/0x48 ext4_kill_sb+0x28/0x60 deactivate_locked_super+0x54/0x130 deactivate_super+0x84/0xa0 cleanup_mnt+0xa4/0x140 __cleanup_mnt+0x18/0x28 task_work_run+0x78/0xe0 do_notify_resume+0x204/0x240 ================================================================== The root cause is a race between cgroup_writeback_umount() and inode_switch_wbs()/cleanup_offline_cgwb(). There is a window between inode_prepare_wbs_switch() returning true and the subsequent wb_queue_isw() call. Following is the process that triggers the issue: CPU A (umount) | CPU B (writeback) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ inode_switch_wbs/cleanup_offline_cgwb atomic_inc(&isw_nr_in_flight) inode_prepare_wbs_switch -> passes SB_ACTIVE check __iget(inode) generic_shutdown_super sb->s_flags &= ~SB_ACTIVE cgroup_writeback_umount(sb) smp_mb() atomic_read(&isw_nr_in_flight) rcu_barrier() -> no pending RCU callbacks flush_workqueue(isw_wq) -> nothing queued, returns evict_inodes(sb) -> Inode skipped as isw still holds a ref. sop->put_super(sb) /* destroys percpu counters */ -> VFS: Busy inodes after unmount! wb_queue_isw() queue_work(isw_wq, ...) /* later in work function */ inode_switch_wbs_work_fn process_inode_switch_wbs iput() -> evict percpu_counter_dec() // UAF! Fix this by extending the RCU read-side critical section in inode_switch_wbs() and cleanup_offline_cgwb() to cover from inode_prepare_wbs_switch() through wb_queue_isw(). Since there is no sleep in this window, rcu_read_lock() can be used. Then add a synchronize_rcu() in cgroup_writeback_umount() before the existing rcu_barrier(), so that all in-flight switchers that have passed the SB_ACTIVE check have completed queue_work() before flush_workqueue() is called. Fixes: a1a0e23e4903 ("writeback: flush inode cgroup wb switches instead of pinning super_block") Suggested-by: Jan Kara Link: https://lore.kernel.org/all/mxnjq2l6guusfchvauxr3v7c4bwjasybxlleqbbh4efloeqspz@iqylk76ohufz Signed-off-by: Baokun Li --- fs/fs-writeback.c | 27 ++++++++++++++++++++++++++- 1 file changed, 26 insertions(+), 1 deletion(-) diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index a65694cbfe68..1f95ddcee363 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -665,7 +665,6 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id) memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys); if (memcg_css && !css_tryget(memcg_css)) memcg_css = NULL; - rcu_read_unlock(); if (!memcg_css) goto out_free; @@ -680,10 +679,18 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id) isw->inodes[0] = inode; trace_inode_switch_wbs_queue(inode->i_wb, new_wb, 1); + /* + * Paired with synchronize_rcu() in cgroup_writeback_umount(). + * Holding rcu_read_lock across wb_queue_isw() ensures + * synchronize_rcu() cannot return until the work is queued, so + * the subsequent flush_workqueue() will wait for the switch. + */ wb_queue_isw(new_wb, isw); + rcu_read_unlock(); return; out_free: + rcu_read_unlock(); atomic_dec(&isw_nr_in_flight); if (new_wb) wb_put(new_wb); @@ -741,6 +748,14 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb) new_wb = &wb->bdi->wb; /* wb_get() is noop for bdi's wb */ nr = 0; + /* + * Paired with synchronize_rcu() in cgroup_writeback_umount(). + * Holding rcu_read_lock across the SB_ACTIVE check, the inode grab + * and wb_queue_isw() ensures synchronize_rcu() cannot return until + * the work is queued, so the subsequent flush_workqueue() will wait + * for the switch. + */ + rcu_read_lock(); spin_lock(&wb->list_lock); /* * In addition to the inodes that have completed writeback, also switch @@ -758,6 +773,7 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb) /* no attached inodes? bail out */ if (nr == 0) { + rcu_read_unlock(); atomic_dec(&isw_nr_in_flight); wb_put(new_wb); kfree(isw); @@ -766,6 +782,7 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb) trace_inode_switch_wbs_queue(wb, new_wb, nr); wb_queue_isw(new_wb, isw); + rcu_read_unlock(); return restart; } @@ -1221,6 +1238,14 @@ void cgroup_writeback_umount(struct super_block *sb) smp_mb(); if (atomic_read(&isw_nr_in_flight)) { + /* + * Paired with rcu_read_lock() in inode_switch_wbs() and + * cleanup_offline_cgwb(). synchronize_rcu() waits for any + * in-flight switcher that already passed the SB_ACTIVE check + * to finish queueing its work, so flush_workqueue() below + * will then drain it. + */ + synchronize_rcu(); /* * Use rcu_barrier() to wait for all pending callbacks to * ensure that all in-flight wb switches are in the workqueue. -- 2.43.7