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

[Bernd Schubert <bernd@bsbernd.com>]

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.