RCU warning off ublk_buf_cleanup() -> mas_for_each()

[Jens Axboe <axboe@kernel.dk>]

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...

-- 
Jens Axboe