A blk-iolatency rq-qos teardown can free struct blk_iolatency while a
freshly queued enable_work callback still references it. The observed
failure is:

blkcg_iolatency_exit() flushes enable_work before deactivating the
iolatency policy. However, blkcg_deactivate_policy() calls
iolatency_pd_offline() for online policy data, and iolatency_pd_offline()
clears min_lat_nsec through iolatency_set_min_lat_nsec(). If this clears
the last nonzero latency target, enable_cnt reaches zero and schedules
enable_work again after the flush has already returned.

The buggy scenario involves two paths, with each column showing the order
within that path:

blkcg_iolatency_exit() path:          system_wq worker path:
1. Flush old enable_work.             1. enable_work is idle.
2. Deactivate the policy.             2. no worker owns it.
3. Offline queues new enable_work.    3. work item becomes pending.
4. Free blkiolat.                     4. worker later runs the item.
5. Owner storage is gone.             5. worker dereferences blkiolat.

Flush enable_work again after blkcg_deactivate_policy() returns and before
freeing blkiolat. Policy offline callbacks have completed at that point,
so the second drain covers the late queueing path without changing the
normal enable/disable accounting rules.

Validation reproduced this kernel report:
BUG: KASAN: slab-use-after-free in assign_work+0x2a/0x150

Call Trace:
 <TASK>
 dump_stack_lvl+0x53/0x70
 print_report+0xd0/0x630
 ? __pfx__raw_spin_lock_irqsave+0x10/0x10
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? __virt_addr_valid+0xea/0x1a0
 ? assign_work+0x2a/0x150
 kasan_report+0xce/0x100
 ? assign_work+0x2a/0x150
 assign_work+0x2a/0x150
 worker_thread+0x1b7/0x500
 ? __pfx_worker_thread+0x10/0x10
 kthread+0x192/0x1d0
 ? __pfx_kthread+0x10/0x10
 ret_from_fork+0x2ac/0x3c0
 ? __pfx_ret_from_fork+0x10/0x10
 ? srso_alias_return_thunk+0x5/0xfbef5
 ? __switch_to+0x2d5/0x6e0
 ? __pfx_kthread+0x10/0x10
 ret_from_fork_asm+0x1a/0x30
 </TASK>

Allocated by task 470:
 kasan_save_stack+0x33/0x60
 kasan_save_track+0x14/0x30
 __kasan_kmalloc+0x8f/0xa0
 iolatency_set_limit+0x301/0x450
 cgroup_file_write+0x178/0x2e0
 kernfs_fop_write_iter+0x1ef/0x290
 vfs_write+0x446/0x6f0
 ksys_write+0xc7/0x160
 do_syscall_64+0xf9/0x540
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Freed by task 611:
 kasan_save_stack+0x33/0x60
 kasan_save_track+0x14/0x30
 kasan_save_free_info+0x3b/0x60
 __kasan_slab_free+0x43/0x70
 kfree+0x131/0x390
 rq_qos_exit+0x5d/0x90
 __del_gendisk+0x394/0x490
 del_gendisk+0xa1/0xe0
 virtblk_remove+0x41/0xd0
 virtio_dev_remove+0x63/0xe0
 device_release_driver_internal+0x246/0x2e0
 unbind_store+0xa9/0xb0
 kernfs_fop_write_iter+0x1ef/0x290
 vfs_write+0x446/0x6f0
 ksys_write+0xc7/0x160
 do_syscall_64+0xf9/0x540
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Last potentially related work creation:
 kasan_save_stack+0x33/0x60
 kasan_record_aux_stack+0x8c/0xa0
 __queue_work+0x42a/0x800
 queue_work_on+0x5d/0x70
 iolatency_set_min_lat_nsec+0x196/0x230
 iolatency_pd_offline+0x1f/0x40
 blkcg_deactivate_policy+0x194/0x270
 blkcg_iolatency_exit+0x33/0x40
 rq_qos_exit+0x5d/0x90
 __del_gendisk+0x394/0x490
 del_gendisk+0xa1/0xe0
 virtblk_remove+0x41/0xd0
 virtio_dev_remove+0x63/0xe0
 device_release_driver_internal+0x246/0x2e0
 unbind_store+0xa9/0xb0
 kernfs_fop_write_iter+0x1ef/0x290
 vfs_write+0x446/0x6f0
 ksys_write+0xc7/0x160
 do_syscall_64+0xf9/0x540
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Second to last potentially related work creation:
 kasan_save_stack+0x33/0x60
 kasan_record_aux_stack+0x8c/0xa0
 __queue_work+0x42a/0x800
 queue_work_on+0x5d/0x70
 iolatency_set_min_lat_nsec+0x196/0x230
 iolatency_set_limit+0x3f1/0x450
 cgroup_file_write+0x178/0x2e0
 kernfs_fop_write_iter+0x1ef/0x290
 vfs_write+0x446/0x6f0
 ksys_write+0xc7/0x160
 do_syscall_64+0xf9/0x540
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fixes: 8a177a3 ("blk-iolatency: Fix inflight count imbalances and IO hangs on offline")
Assisted-by: Codex:gpt-5.5
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>