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

[Jens Axboe <axboe@kernel.dk>]

On 4/23/26 3:51 PM, Bernd Schubert wrote:
> 
> 
> On 4/23/26 23:48, Jens Axboe wrote:
>> 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
>>
> 
> Thank you, will check after getting some sleep.

Sounds good - fixes the issue on my end.

-- 
Jens Axboe