From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 6E7CC48B39E for ; Tue, 14 Jul 2026 14:50:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784040645; cv=none; b=ApZQtllMmGbyqaZrKXJVHNBN6/JKnxFb7DCgMYsyfz0knSxAb5Q0S3o0AxVIAAiBWLo0B3S+1mxjUCMnLWIyj19FN8BOd4BWrj/SYt1TL7uNqpNq9DJiehpOnJ9FeY71Fi5mlcK4tFBMG4vvf0UcfJuy+uRbaXLYv/w/zJvhr9M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784040645; c=relaxed/simple; bh=iEume0c4XpegVLi8PuDhYZp3+EzOlfh5ugWF52XOMCg=; h=Subject:To:Cc:From:Date:Message-ID:MIME-Version:Content-Type; b=Q1XNX542FQPOgheeqzZqTC4aDlIa/RPbdyG5HsxsSpMLCvCvzoFNwdyzDBFOiNDDG0avz2UxLVAG2kJPv7Ub3Q9/IfbK9FRcDsTYVJliIdQH40ixdt0qQE3W5hkS3C4DJevoB4QqvWqu9qaHtiOEhDDmGuGqG+kz+N8TgcOh2xg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=JZjYzRBL; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="JZjYzRBL" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7DDE61F000E9; Tue, 14 Jul 2026 14:50:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1784040636; bh=4yh7LQZvCAxvVIk80+psNiDNNbvCL/U2VsxZJ18bDjQ=; h=Subject:To:Cc:From:Date; b=JZjYzRBL3pHAAjfBULX3/AIxieq/Mme5XckXNKaEY7xILpY7d5YRIXg/mE01mP4Ks v9Fglf07BkTyk8gSGMP2ESxEiXLXgzPmrwXYc16JCmsOOpXffjPiFcUJvQPyklI3yO gPq4u+IdK2Xld4f8gMcUK2qercPMYf0Sjq8lC/bw= Subject: FAILED: patch "[PATCH] writeback: fix race between cgroup_writeback_umount() and" failed to apply to 5.15-stable tree To: libaokun@linux.alibaba.com,brauner@kernel.org,jack@suse.cz,tj@kernel.org Cc: From: Date: Tue, 14 Jul 2026 16:48:47 +0200 Message-ID: <2026071447-activism-denture-ed03@gregkh> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=ANSI_X3.4-1968 Content-Transfer-Encoding: 8bit The patch below does not apply to the 5.15-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to . To reproduce the conflict and resubmit, you may use the following commands: git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y git checkout FETCH_HEAD git cherry-pick -x cba38ec4cbd3a7b8b942a8d52531a05be8a9ff0d # git commit -s git send-email --to '' --in-reply-to '2026071447-activism-denture-ed03@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..' Possible dependencies: thanks, greg k-h ------------------ original commit in Linus's tree ------------------ >From cba38ec4cbd3a7b8b942a8d52531a05be8a9ff0d Mon Sep 17 00:00:00 2001 From: Baokun Li Date: Thu, 21 May 2026 17:50:14 +0800 Subject: [PATCH] writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs() 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. The existing rcu_barrier() is intentionally retained so this fix can be backported unchanged to stable kernels (5.10.y, 6.6.y, ...) that still queue switches via queue_rcu_work(). It is a no-op on current mainline (since commit e1b849cfa6b6 ("writeback: Avoid contention on wb->list_lock when switching inodes")) and is removed in a follow-up patch. Fixes: a1a0e23e4903 ("writeback: flush inode cgroup wb switches instead of pinning super_block") Cc: stable@vger.kernel.org Suggested-by: Jan Kara Link: https://lore.kernel.org/all/mxnjq2l6guusfchvauxr3v7c4bwjasybxlleqbbh4efloeqspz@iqylk76ohufz Reviewed-by: Jan Kara Signed-off-by: Baokun Li Link: https://patch.msgid.link/20260521095016.2791354-2-libaokun@linux.alibaba.com Acked-by: Tejun Heo Signed-off-by: Christian Brauner (Amutable) diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index a65694cbfe68..6766de9f9d75 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -660,12 +660,19 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id) atomic_inc(&isw_nr_in_flight); - /* find and pin the new wb */ + /* + * Paired with synchronize_rcu() in cgroup_writeback_umount(): + * holding rcu_read_lock across inode_prepare_wbs_switch() + * (covering the SB_ACTIVE check and 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(); + /* find and pin the new wb */ 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; @@ -681,9 +688,11 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id) trace_inode_switch_wbs_queue(inode->i_wb, new_wb, 1); 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 +750,14 @@ 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 +775,7 @@ 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 +784,7 @@ 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 +1240,14 @@ 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.