From cba38ec4cbd3a7b8b942a8d52531a05be8a9ff0d Mon Sep 17 00:00:00 2001 From: Baokun Li Date: Thu, 21 May 2026 17:50:14 +0800 Subject: 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) --- fs/fs-writeback.c | 31 +++++++++++++++++++++++++++++-- 1 file changed, 29 insertions(+), 2 deletions(-) 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. -- cgit v1.2.3 From e90a6d668e26e00a72df2d09c173b563468f09c9 Mon Sep 17 00:00:00 2001 From: Baokun Li Date: Thu, 21 May 2026 17:50:15 +0800 Subject: writeback: drop now-unnecessary rcu_barrier() in cgroup_writeback_umount() Commit e1b849cfa6b6 ("writeback: Avoid contention on wb->list_lock when switching inodes") replaced the queue_rcu_work() based scheduling of inode wb switches with a plain queue_work(). Since then no switcher goes through call_rcu(), so rcu_barrier() in cgroup_writeback_umount() has no callbacks of its own to wait for. It still drains unrelated call_rcu() callbacks from other subsystems on busy systems, which incidentally slows umount down; drop it. Fixes: e1b849cfa6b6 ("writeback: Avoid contention on wb->list_lock when switching inodes") Reviewed-by: Jan Kara Signed-off-by: Baokun Li Link: https://patch.msgid.link/20260521095016.2791354-3-libaokun@linux.alibaba.com Acked-by: Tejun Heo Signed-off-by: Christian Brauner (Amutable) --- fs/fs-writeback.c | 5 ----- 1 file changed, 5 deletions(-) diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index 6766de9f9d75..325a30cc35bf 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -1248,11 +1248,6 @@ void cgroup_writeback_umount(struct super_block *sb) * 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. - */ - rcu_barrier(); flush_workqueue(isw_wq); } } -- cgit v1.2.3 From 31c1d19ead2c26a63859a2757d8b786765ba9cdd Mon Sep 17 00:00:00 2001 From: Baokun Li Date: Thu, 21 May 2026 17:50:16 +0800 Subject: writeback: use a per-sb counter to drain inode wb switches at umount Tracking in-flight inode wb switches with a single global counter (isw_nr_in_flight) plus a synchronize_rcu() based wait in cgroup_writeback_umount() forces every umount to take a global hit whenever any other superblock on the system has wb switches in flight, even if the superblock being unmounted has none of its own. Replace the global synchronize_rcu()/flush_workqueue() pair with a per-sb counter, s_isw_nr_in_flight, plus three small helpers: - cgroup_writeback_pin(sb) - increment counter - cgroup_writeback_unpin(sb) - decrement and wake drainer if last - cgroup_writeback_drain(sb) - wait for counter to reach zero The wiring is: - inode_prepare_wbs_switch() pins before checking SB_ACTIVE and grabbing the inode; failure paths unpin before returning. A lockless SB_ACTIVE check at the top of the function lets us skip the atomic_inc/smp_mb dance once SB_ACTIVE has been cleared (it is monotonic and never set back). - process_inode_switch_wbs() unpins after the matching iput(). - cgroup_writeback_umount() drains the per-sb counter via wait_var_event(). The smp_mb() pair between inode_prepare_wbs_switch() and cgroup_writeback_umount() keeps the SB_ACTIVE / counter ordering: either the umounter sees a non-zero counter and waits, or the switcher sees SB_ACTIVE cleared and aborts before grabbing the inode. The global isw_nr_in_flight is left in place, since it is still used to throttle in-flight switches via WB_FRN_MAX_IN_FLIGHT. The rcu_read_lock() extension in inode_switch_wbs() and cleanup_offline_cgwb() that the race fix added is no longer needed and is reverted; the synchronize_rcu() that the race fix added to cgroup_writeback_umount() is dropped as well. The following numbers were measured on a 16 vCPU QEMU guest with 4 background superblocks each churning "create memcg -> write 1 MiB -> rmdir memcg" to keep the global isw_nr_in_flight non-zero. Latencies are wall-clock around umount(8); only the target sb's umount is measured. Target sb runs its own cgwb churn: p50 p95 p99 max global synchronize_rcu() 67.6 ms 88.3 ms 88.3 ms 96.8 ms per-sb counter (this) 7.9 ms 10.0 ms 10.0 ms 10.1 ms Idle target umount latency under cross-sb cgwb-switch pressure: p50 p95 p99 max global synchronize_rcu() 62.7 ms 95.4 ms 108.1 ms 108.6 ms per-sb counter (this) 5.3 ms 6.9 ms 7.4 ms 7.4 ms no-pressure baseline 4.9 ms 5.9 ms 6.3 ms 6.7 ms 8 concurrent umounts of idle sbs under the same pressure: p50 p95 max global synchronize_rcu() 61.3 ms 99.5 ms 113.7 ms per-sb counter (this) 8.1 ms 9.1 ms 9.5 ms In-kernel cgroup_writeback_umount() time across the same run (bpftrace, ~340 calls covering all scenarios): global synchronize_rcu() 12371 ms total (~36 ms / call) per-sb counter (this) 1.37 ms total ( ~4 us / call) Suggested-by: Christian Brauner Link: https://lore.kernel.org/r/177910456953.488929.2169908940676707307.b4-review@b4 Reviewed-by: Jan Kara Signed-off-by: Baokun Li Link: https://patch.msgid.link/20260521095016.2791354-4-libaokun@linux.alibaba.com Acked-by: Tejun Heo Signed-off-by: Christian Brauner (Amutable) --- fs/fs-writeback.c | 97 ++++++++++++++++++++---------------------- include/linux/fs/super_types.h | 8 ++++ 2 files changed, 55 insertions(+), 50 deletions(-) diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index 325a30cc35bf..900ad7818bd4 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -497,6 +497,23 @@ skip_switch: return switched; } +static inline void cgroup_writeback_pin(struct super_block *sb) +{ + atomic_inc(&sb->s_isw_nr_in_flight); +} + +static inline void cgroup_writeback_unpin(struct super_block *sb) +{ + if (atomic_dec_and_test(&sb->s_isw_nr_in_flight)) + wake_up_var(&sb->s_isw_nr_in_flight); +} + +static inline void cgroup_writeback_drain(struct super_block *sb) +{ + wait_var_event(&sb->s_isw_nr_in_flight, + !atomic_read(&sb->s_isw_nr_in_flight)); +} + static void process_inode_switch_wbs(struct bdi_writeback *new_wb, struct inode_switch_wbs_context *isw) { @@ -554,8 +571,12 @@ relock: wb_put_many(old_wb, nr_switched); } - for (inodep = isw->inodes; *inodep; inodep++) + for (inodep = isw->inodes; *inodep; inodep++) { + struct super_block *sb = (*inodep)->i_sb; + iput(*inodep); + cgroup_writeback_unpin(sb); + } wb_put(new_wb); kfree(isw); atomic_dec(&isw_nr_in_flight); @@ -598,16 +619,19 @@ void inode_switch_wbs_work_fn(struct work_struct *work) static bool inode_prepare_wbs_switch(struct inode *inode, struct bdi_writeback *new_wb) { + /* Avoid the atomic_inc/smp_mb dance once SB_ACTIVE is gone. */ + if (!(inode->i_sb->s_flags & SB_ACTIVE)) + return false; + /* - * Paired with smp_mb() in cgroup_writeback_umount(). - * isw_nr_in_flight must be increased before checking SB_ACTIVE and - * grabbing an inode, otherwise isw_nr_in_flight can be observed as 0 - * in cgroup_writeback_umount() and the isw_wq will be not flushed. + * Pairs with smp_mb() in cgroup_writeback_umount(): the umounter either + * sees a non-zero counter and waits, or we see SB_ACTIVE clear below. */ + cgroup_writeback_pin(inode->i_sb); smp_mb(); if (IS_DAX(inode)) - return false; + goto out_unpin; /* while holding I_WB_SWITCH, no one else can update the association */ spin_lock(&inode->i_lock); @@ -615,13 +639,17 @@ static bool inode_prepare_wbs_switch(struct inode *inode, inode_state_read(inode) & (I_WB_SWITCH | I_FREEING | I_WILL_FREE) || inode_to_wb(inode) == new_wb) { spin_unlock(&inode->i_lock); - return false; + goto out_unpin; } inode_state_set(inode, I_WB_SWITCH); __iget(inode); spin_unlock(&inode->i_lock); return true; + +out_unpin: + cgroup_writeback_unpin(inode->i_sb); + return false; } static void wb_queue_isw(struct bdi_writeback *wb, @@ -660,19 +688,12 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id) atomic_inc(&isw_nr_in_flight); - /* - * 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 */ + rcu_read_lock(); 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; @@ -688,11 +709,9 @@ 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); @@ -750,14 +769,6 @@ 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 @@ -775,7 +786,6 @@ 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); @@ -784,7 +794,6 @@ 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; } @@ -1217,39 +1226,27 @@ out_bdi_put: } /** - * cgroup_writeback_umount - flush inode wb switches for umount + * cgroup_writeback_umount - wait for in-flight inode wb switches on @sb * @sb: target super_block * - * This function is called when a super_block is about to be destroyed and - * flushes in-flight inode wb switches. An inode wb switch goes through - * RCU and then workqueue, so the two need to be flushed in order to ensure - * that all previously scheduled switches are finished. As wb switches are - * rare occurrences and synchronize_rcu() can take a while, perform - * flushing iff wb switches are in flight. + * Wait until every inode wb switch that already passed the SB_ACTIVE + * check on this superblock has been completed by the worker. Since + * SB_ACTIVE is cleared before this is called, no new switches can start + * for @sb, so s_isw_nr_in_flight will monotonically drop to zero. */ void cgroup_writeback_umount(struct super_block *sb) { - if (!(sb->s_bdi->capabilities & BDI_CAP_WRITEBACK)) return; /* - * SB_ACTIVE should be reliably cleared before checking - * isw_nr_in_flight, see generic_shutdown_super(). + * Pairs with smp_mb() in inode_prepare_wbs_switch(): we either observe + * a non-zero counter and wait, or the switcher sees SB_ACTIVE clear + * (cleared by generic_shutdown_super()) and bails before grabbing the + * inode. */ 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(); - flush_workqueue(isw_wq); - } + cgroup_writeback_drain(sb); } static int __init cgroup_writeback_init(void) diff --git a/include/linux/fs/super_types.h b/include/linux/fs/super_types.h index 383050e7fdf5..1ab4e2265129 100644 --- a/include/linux/fs/super_types.h +++ b/include/linux/fs/super_types.h @@ -274,6 +274,14 @@ struct super_block { /* number of fserrors that are being sent to fsnotify/filesystems */ refcount_t s_pending_errors; + +#ifdef CONFIG_CGROUP_WRITEBACK + /* + * Number of in-flight inode wb switches for this sb. Drained by + * cgroup_writeback_umount() before tear-down. + */ + atomic_t s_isw_nr_in_flight; +#endif } __randomize_layout; /* -- cgit v1.2.3