On 9/23/25 1:25 PM, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> Following deadlock can be triggered easily by lockdep:
>
> WARNING: possible circular locking dependency detected
> 6.17.0-rc3-00124-ga12c2658ced0 #1665 Not tainted
> ------------------------------------------------------
> check/1334 is trying to acquire lock:
> ff1100011d9d0678 (&q->sysfs_lock){+.+.}-{4:4}, at: blk_unregister_queue+0x53/0x180
>
> but task is already holding lock:
> ff1100011d9d00e0 (&q->q_usage_counter(queue)#3){++++}-{0:0}, at: del_gendisk+0xba/0x110
>
> which lock already depends on the new lock.
>
> the existing dependency chain (in reverse order) is:
>
> -> #2 (&q->q_usage_counter(queue)#3){++++}-{0:0}:
> blk_queue_enter+0x40b/0x470
> blkg_conf_prep+0x7b/0x3c0
> tg_set_limit+0x10a/0x3e0
> cgroup_file_write+0xc6/0x420
> kernfs_fop_write_iter+0x189/0x280
> vfs_write+0x256/0x490
> ksys_write+0x83/0x190
> __x64_sys_write+0x21/0x30
> x64_sys_call+0x4608/0x4630
> do_syscall_64+0xdb/0x6b0
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
>
> -> #1 (&q->rq_qos_mutex){+.+.}-{4:4}:
> __mutex_lock+0xd8/0xf50
> mutex_lock_nested+0x2b/0x40
> wbt_init+0x17e/0x280
> wbt_enable_default+0xe9/0x140
> blk_register_queue+0x1da/0x2e0
> __add_disk+0x38c/0x5d0
> add_disk_fwnode+0x89/0x250
> device_add_disk+0x18/0x30
> virtblk_probe+0x13a3/0x1800
> virtio_dev_probe+0x389/0x610
> really_probe+0x136/0x620
> __driver_probe_device+0xb3/0x230
> driver_probe_device+0x2f/0xe0
> __driver_attach+0x158/0x250
> bus_for_each_dev+0xa9/0x130
> driver_attach+0x26/0x40
> bus_add_driver+0x178/0x3d0
> driver_register+0x7d/0x1c0
> __register_virtio_driver+0x2c/0x60
> virtio_blk_init+0x6f/0xe0
> do_one_initcall+0x94/0x540
> kernel_init_freeable+0x56a/0x7b0
> kernel_init+0x2b/0x270
> ret_from_fork+0x268/0x4c0
> ret_from_fork_asm+0x1a/0x30
>
> -> #0 (&q->sysfs_lock){+.+.}-{4:4}:
> __lock_acquire+0x1835/0x2940
> lock_acquire+0xf9/0x450
> __mutex_lock+0xd8/0xf50
> mutex_lock_nested+0x2b/0x40
> blk_unregister_queue+0x53/0x180
> __del_gendisk+0x226/0x690
> del_gendisk+0xba/0x110
> sd_remove+0x49/0xb0 [sd_mod]
> device_remove+0x87/0xb0
> device_release_driver_internal+0x11e/0x230
> device_release_driver+0x1a/0x30
> bus_remove_device+0x14d/0x220
> device_del+0x1e1/0x5a0
> __scsi_remove_device+0x1ff/0x2f0
> scsi_remove_device+0x37/0x60
> sdev_store_delete+0x77/0x100
> dev_attr_store+0x1f/0x40
> sysfs_kf_write+0x65/0x90
> kernfs_fop_write_iter+0x189/0x280
> vfs_write+0x256/0x490
> ksys_write+0x83/0x190
> __x64_sys_write+0x21/0x30
> x64_sys_call+0x4608/0x4630
> do_syscall_64+0xdb/0x6b0
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
>
> other info that might help us debug this:
>
> Chain exists of:
> &q->sysfs_lock --> &q->rq_qos_mutex --> &q->q_usage_counter(queue)#3
>
> Possible unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(&q->q_usage_counter(queue)#3);
> lock(&q->rq_qos_mutex);
> lock(&q->q_usage_counter(queue)#3);
> lock(&q->sysfs_lock);
>
> Root cause is that queue_usage_counter is grabbed with rq_qos_mutex
> held in blkg_conf_prep(), while queue should be freezed before
> rq_qos_mutex from other context.
>
> The blk_queue_enter() from blkg_conf_prep() is used to protect against
> policy deactivation, which is already protected with blkcg_mutex, hence
> convert blk_queue_enter() to blkcg_mutex to fix this problem. Meanwhile,
> consider that blkcg_mutex is held after queue is freezed from policy
> deactivation, also convert blkg_alloc() to use GFP_NOIO.
>
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
Looks good to me:
Reviewed-by : Nilay Shroff <nilay@xxxxxxxxxxxxx>
