Re: RCU warning off ublk_buf_cleanup() -> mas_for_each()

Liam R. Howlett
2026-04-21T20:29:05Z

* Jens Axboe <axboe@kernel.dk> [260421 13:47]:
> Hi Ming,
>
> Ran into the below running tests on the current tree:
>
> =============================
> WARNING: suspicious RCU usage
> 7.0.0+ #16 Tainted: G N
> -----------------------------
> lib/maple_tree.c:759 suspicious rcu_dereference_check() usage!
>
> other info that might help us debug this:
>
>
> rcu_scheduler_active = 2, debug_locks = 1
> 1 lock held by iou-wrk-55535/55536:
> #0: ffff800085a451a0 (ublk_ctl_mutex){+.+.}-{4:4}, at: ublk_ctrl_del_dev+0xdc/0x2f8
>
> stack backtrace:
> CPU: 4 UID: 0 PID: 55536 Comm: iou-wrk-55535 Tainted: G N 7.0.0+ #16 PREEMPT
> Tainted: [N]=TEST
> Hardware name: linux,dummy-virt (DT)
> Call trace:
> show_stack+0x1c/0x30 (C)
> dump_stack_lvl+0x68/0x90
> dump_stack+0x18/0x20
> lockdep_rcu_suspicious+0x170/0x200
> mas_walk+0x3f0/0x6a0
> mas_find+0x1b4/0x6b0
> ublk_buf_cleanup+0xe0/0x240
> ublk_cdev_rel+0x34/0x1b0
> device_release+0xa4/0x350
> kobject_put+0x138/0x250
> put_device+0x18/0x30
> ublk_put_device+0x18/0x28
> ublk_ctrl_del_dev+0x120/0x2f8
> ublk_ctrl_uring_cmd+0x598/0x29b8
> io_uring_cmd+0x1e0/0x468
> __io_issue_sqe+0xa4/0x748
> io_issue_sqe+0x80/0xf68
> io_wq_submit_work+0x26c/0xdc8
> io_worker_handle_work+0x334/0xf20
> io_wq_worker+0x278/0x9e8
> ret_from_fork+0x10/0x20
> Buffer I/O error on dev ublkb0, logical block 0, async page read
> Buffer I/O error on dev ublkb0, logical block 0, async page read
> ublkb0: unable to read partition table
> Buffer I/O error on dev ublkb0, logical block 0, async page read
> Buffer I/O error on dev ublkb0, logical block 0, async page read
> Buffer I/O error on dev ublkb0, logical block 512, async page read
> Buffer I/O error on dev ublkb0, logical block 512, async page read
> Buffer I/O error on dev ublkb0, logical block 0, async page read
> Buffer I/O error on dev ublkb0, logical block 512, async page read
>
> and I briefly looked at it, but then just gave up as a) the maple tree
> documentation is not that detailed,

Which documentation is lacking? I will fix it.

I have user documentation in the Documentation directory while
technical details are in the code.

>and b) other in-tree users also just
> call mas_for_each() without either a lock held or RCU read side locked.

mas_for_each() must hold a lock of some type.

>
> Adding Liam for shedding some light on this...
>
> --
> Jens Axboe
>