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

Bernd Schubert
2026-04-23T21:51:50Z

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.