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

[Jens Axboe <axboe@kernel.dk>]

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