From: Tao Cui loop_clear_limits() calls queue_limits_commit_update() directly from the loop workqueue that processes the request. That does a non-atomic struct assignment to q->limits without freezing the queue, which races with lockless readers of q->limits on other CPUs - bio splitting reads max_hw_sectors, the discard path reads max_hw_discard_sectors - and can let them observe torn values. The trigger is a discard or write-zeroes request on a loop device whose backing file does not support the corresponding fallocate operation. The code already has an XXX comment saying this should move to a workqueue. Do that: schedule a work item on the system workqueue, where it is safe to freeze the queue and update the limits using queue_limits_commit_update_frozen(). Accumulate pending modes in lo->clear_limits_mode so that failures between scheduling and execution of the work item are not lost, reset the accumulated modes when a new backing file is assigned, and cancel the work item before the device is freed. Signed-off-by: Tao Cui --- Changes since v1: - Make clear_limits_mode atomic_t: loop workers can hit the |= concurrently. - Reset clear_limits_mode when assigning a new backing file, so stale modes do not clear limits of the new file. - Cancel the work item from loop_remove() before del_gendisk(): the queue can already be in RCU-delayed freeing when lo_free_disk() cancels it. Tested on x86-64 (qemu, vfat-backed loop device): 30s discard and reconfigure loop exercises the clear 577 times, no torn sysfs reads, no difference against the unpatched kernel. Link: https://lore.kernel.org/r/20260828072004.273519-1-cui.tao@linux.dev/ --- drivers/block/loop.c | 29 ++++++++++++++++++++--------- 1 file changed, 20 insertions(+), 9 deletions(-) diff --git a/drivers/block/loop.c b/drivers/block/loop.c index 6f12976035b0..368ea857895c 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -67,6 +67,8 @@ struct loop_device { struct list_head rootcg_cmd_list; struct list_head idle_worker_list; struct rb_root worker_tree; + struct work_struct clear_limits_work; + atomic_t clear_limits_mode; struct timer_list timer; bool sysfs_inited; @@ -222,9 +224,12 @@ static void loop_set_size(struct loop_device *lo, loff_t size) kobject_uevent(&disk_to_dev(lo->lo_disk)->kobj, KOBJ_CHANGE); } -static void loop_clear_limits(struct loop_device *lo, int mode) +static void loop_clear_limits_workfn(struct work_struct *work) { + struct loop_device *lo = + container_of(work, struct loop_device, clear_limits_work); struct queue_limits lim = queue_limits_start_update(lo->lo_queue); + int mode = atomic_xchg(&lo->clear_limits_mode, 0); if (mode & FALLOC_FL_ZERO_RANGE) lim.max_write_zeroes_sectors = 0; @@ -234,14 +239,13 @@ static void loop_clear_limits(struct loop_device *lo, int mode) lim.discard_granularity = 0; } - /* - * XXX: this updates the queue limits without freezing the queue, which - * is against the locking protocol and dangerous. But we can't just - * freeze the queue as we're inside the ->queue_rq method here. So this - * should move out into a workqueue unless we get the file operations to - * advertise if they support specific fallocate operations. - */ - queue_limits_commit_update(lo->lo_queue, &lim); + queue_limits_commit_update_frozen(lo->lo_queue, &lim); +} + +static void loop_clear_limits(struct loop_device *lo, int mode) +{ + atomic_or(mode, &lo->clear_limits_mode); + schedule_work(&lo->clear_limits_work); } static int lo_fallocate(struct loop_device *lo, struct request *rq, loff_t pos, @@ -516,6 +520,7 @@ static int loop_validate_file(struct file *file, struct block_device *bdev) static void loop_assign_backing_file(struct loop_device *lo, struct file *file) { lo->lo_backing_file = file; + atomic_set(&lo->clear_limits_mode, 0); lo->old_gfp_mask = mapping_gfp_mask(file->f_mapping); mapping_set_gfp_mask(file->f_mapping, lo->old_gfp_mask & ~(__GFP_IO | __GFP_FS)); @@ -1781,6 +1786,7 @@ static void lo_free_disk(struct gendisk *disk) destroy_workqueue(lo->workqueue); loop_free_idle_workers(lo, true); timer_shutdown_sync(&lo->timer); + cancel_work_sync(&lo->clear_limits_work); mutex_destroy(&lo->lo_mutex); kfree(lo); } @@ -2100,6 +2106,7 @@ static int loop_add(int i) spin_lock_init(&lo->lo_lock); spin_lock_init(&lo->lo_work_lock); INIT_WORK(&lo->rootcg_work, loop_rootcg_workfn); + INIT_WORK(&lo->clear_limits_work, loop_clear_limits_workfn); INIT_LIST_HEAD(&lo->rootcg_cmd_list); disk->major = LOOP_MAJOR; disk->first_minor = i << part_shift; @@ -2138,6 +2145,10 @@ static int loop_add(int i) static void loop_remove(struct loop_device *lo) { + /* Cancel early: the queue may be in RCU-delayed freeing + * by the time lo_free_disk() runs. */ + cancel_work_sync(&lo->clear_limits_work); + /* Make this loop device unreachable from pathname. */ del_gendisk(lo->lo_disk); blk_mq_free_tag_set(&lo->tag_set); -- 2.43.0