WARNING: possible circular locking dependency detected in nbd

[Sven Schnelle]

Hi,

i'm seeing the lockdep splat below in CI. I think this is because
nbd_open is called with disk->open_mutex held, and acquires
nbd_index_mutex. However, nbd_put() first takes the nbd_index_lock,
and calls del_gendisk, which locks disk->open_mutex, so the order is
reversed.

WARNING: possible circular locking dependency detected
5.14.0-20210816.rc5.git0.04a03f7da6c2.300.fc34.s390x+debug #1 Not tainted
------------------------------------------------------
modprobe/17864 is trying to acquire lock:
00000001dea24d28 (&disk->open_mutex){+.+.}-{3:3}, at: del_gendisk+0x64/0x210

but task is already holding lock:
000003ff805fd6e8 (nbd_index_mutex){+.+.}-{3:3}, at: refcount_dec_and_mutex_lock+0x7e/0x110

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:
-> #1 (nbd_index_mutex){+.+.}-{3:3}:
       validate_chain+0x9ca/0xde8
       __lock_acquire+0x64c/0xc40
       lock_acquire.part.0+0xec/0x258
       lock_acquire+0xb0/0x200
       __mutex_lock+0xa2/0x8d8
       mutex_lock_nested+0x32/0x40
       nbd_open+0x30/0x248 [nbd]
       blkdev_get_whole+0x38/0x128
       blkdev_get_by_dev+0xcc/0x400
       blkdev_open+0x7a/0xd8
       do_dentry_open+0x19e/0x390
       do_open+0x2e0/0x458
       path_openat+0xec/0x2a8
       do_filp_open+0x90/0x130
       do_sys_openat2+0xa8/0x168
       do_sys_open+0x62/0x90
       __do_syscall+0x1c2/0x1f0
       system_call+0x78/0xa0

-> #0 (&disk->open_mutex){+.+.}-{3:3}:
       check_noncircular+0x168/0x188
       check_prev_add+0xe0/0xed8
       validate_chain+0x9ca/0xde8
       __lock_acquire+0x64c/0xc40
       lock_acquire.part.0+0xec/0x258
       lock_acquire+0xb0/0x200
       __mutex_lock+0xa2/0x8d8
       mutex_lock_nested+0x32/0x40
       del_gendisk+0x64/0x210
       nbd_put.part.0+0x46/0x98 [nbd]
       nbd_cleanup+0xde/0x118 [nbd]
       __do_sys_delete_module+0x19a/0x2a8
       __do_syscall+0x1c2/0x1f0
       system_call+0x78/0xa0

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(nbd_index_mutex);
                               lock(&disk->open_mutex);
                               lock(nbd_index_mutex);
  lock(&disk->open_mutex);

 *** DEADLOCK ***

1 lock held by modprobe/17864:
 #0: 000003ff805fd6e8 (nbd_index_mutex){+.+.}-{3:3}, at: refcount_dec_and_mutex_lock+0x7e/0x110

stack backtrace:
CPU: 1 PID: 17864 Comm: modprobe Not tainted 5.14.0-20210816.rc5.git0.04a03f7da6c2.300.fc34.s390x+debug #1
Hardware name: IBM 8561 T01 703 (LPAR)
Call Trace:
 [<000000008f735098>] show_stack+0x90/0xf8 
 [<000000008f746d56>] dump_stack_lvl+0x8e/0xc8 
 [<000000008eb3a3b0>] check_noncircular+0x168/0x188 
 [<000000008eb3b470>] check_prev_add+0xe0/0xed8 
 [<000000008eb3cc32>] validate_chain+0x9ca/0xde8 
 [<000000008eb3fc2c>] __lock_acquire+0x64c/0xc40 
 [<000000008eb3e834>] lock_acquire.part.0+0xec/0x258 
 [<000000008eb3ea50>] lock_acquire+0xb0/0x200 
 [<000000008f75591a>] __mutex_lock+0xa2/0x8d8 
 [<000000008f756182>] mutex_lock_nested+0x32/0x40 
 [<000000008f2538cc>] del_gendisk+0x64/0x210 
 [<000003ff805f4936>] nbd_put.part.0+0x46/0x98 [nbd] 
 [<000003ff805f965e>] nbd_cleanup+0xde/0x118 [nbd] 
 [<000000008eba7a52>] __do_sys_delete_module+0x19a/0x2a8 
 [<000000008f74a67a>] __do_syscall+0x1c2/0x1f0 
 [<000000008f75cf38>] system_call+0x78/0xa0 
INFO: lockdep is turned off.