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

Jens Axboe
2026-04-23T21:48:46Z

On 4/23/26 3:45 PM, Bernd Schubert wrote:
>
>
> On 4/21/26 19:47, Jens Axboe wrote:
>> 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, and b) other in-tree users also just
>> call mas_for_each() without either a lock held or RCU read side locked.
>>
>> Adding Liam for shedding some light on this...
>>
>
> Hmm, that is another one, I run into
>
> Date: Thu Apr 16 15:16:04 2026 +0200
>
> ublk: Add rcu_read_lock/unlock in ublk_buf_cleanup
>
> [ 399.994025] =============================
> [ 399.994694] WARNING: suspicious RCU usage
> [ 399.995464] 7.0.0+ #52 Not tainted
> [ 399.996034] -----------------------------
> [ 399.996697] lib/maple_tree.c:780 suspicious rcu_dereference_check() usage!
> [ 399.997748]
> other info that might help us debug this:
>
> [ 399.998961]
> rcu_scheduler_active = 2, debug_locks = 1
> [ 399.999957] 1 lock held by iou-wrk-1495/1509:
> [ 400.000596] #0: ffffffffa05e4590 (ublk_ctl_mutex){+.+.}-{4:4}, at: ublk_ctrl_del_dev+0x3d/0x190 [ublk_drv]
> [ 400.001796]
> stack backtrace:
> [ 400.002492] CPU: 40 UID: 0 PID: 1509 Comm: iou-wrk-1495 Not tainted 7.0.0+ #52 PREEMPT
> [ 400.002496] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.17.0-debian-1.17.0-1ubuntu1 04/01/2014
> [ 400.002498] Call Trace:
> [ 400.002500] <TASK>
> [ 400.002503] dump_stack_lvl+0x54/0x70
> [ 400.002511] lockdep_rcu_suspicious+0x119/0x160
> [ 400.002521] mas_start+0xed/0x140
> [ 400.002531] mas_find+0x1a6/0x270
> [ 400.002538] ublk_cdev_rel+0xb8/0x320 [ublk_drv]
>
>
> And have in my pbuf branch
>
> index eb9b5dd8122c..208f6b5ad892 100644
> --- a/drivers/block/ublk_drv.c
> +++ b/drivers/block/ublk_drv.c
> @@ -5478,6 +5478,7 @@ static void ublk_buf_cleanup(struct ublk_device *ub)
> struct ublk_buf_range *range;
> struct page *pages[32];
>
> + rcu_read_lock();
> mas_for_each(&mas, range, ULONG_MAX) {
> unsigned long base = mas.index;
> unsigned long nr = mas.last - base + 1;
> @@ -5495,6 +5496,7 @@ static void ublk_buf_cleanup(struct ublk_device *ub)
> }
> kfree(range);
> }
> + rcu_read_unlock();
> mtree_destroy(&ub->buf_tree);
> ida_destroy(&ub->buf_ida);
> }

Fixed in patches going to Linus tomorrow, see HEAD~3..HEAD~1 in this
branch:

https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux.git/log/?h=block-7.1

--
Jens Axboe