lockdep warning while reading sysfs

[Qian Cai <cai@lca.pw>]

LTP: starting read_all_sys (read_all -d /sys -q -r 10 -e /sys/power/wakeup_count)

Suppose this simply by reading files in /sys/kernel/slab/* would trigger this.
Basically, it acquired kn->count#69 in kernfs_seq_start():

mutex_lock(&of->mutex);
if (!kernfs_get_active(of->kn))

in kernfs_get_active():

if (kernfs_lockdep(kn))
	rwsem_acquire_read(&kn->dep_map, 0, 1, _RET_IP_);

Then, it will acquires mem_hotplug_lock.rw_sem in show_slab_objects() ->
get_online_mems()

Then, another CPU acquired mem_hotplug_lock.rw_sem, and then calls
secondary_startup() I guess it it from the CPU hotplug path to trigger a deadlock.

======================================================
WARNING: possible circular locking dependency detected
5.0.0-rc1+ #60 Not tainted
------------------------------------------------------
read_all/7952 is trying to acquire lock:
0000000019f12603 (mem_hotplug_lock.rw_sem){++++}, at: show_slab_objects+0x16c/0x450

but task is already holding lock:
000000008804717f (kn->count#69){++++}, at: kernfs_seq_start+0x79/0x170

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #3 (kn->count#69){++++}:
       __lock_acquire+0x728/0x1200
       lock_acquire+0x269/0x5a0
       __kernfs_remove+0x72f/0x9a0
       kernfs_remove_by_name_ns+0x45/0x90
       sysfs_remove_link+0x3c/0xa0
       sysfs_slab_add+0x1bd/0x330
       __kmem_cache_create+0x166/0x1c0
       create_cache+0xcf/0x1f0
       kmem_cache_create_usercopy+0x1aa/0x270
       kmem_cache_create+0x16/0x20
       mlx5_init_fs+0x195/0x1a10 [mlx5_core]
       mlx5_load_one+0x1106/0x1e90 [mlx5_core]
       init_one+0x864/0xd60 [mlx5_core]
       local_pci_probe+0xda/0x190
       work_for_cpu_fn+0x56/0xa0
       process_one_work+0xad7/0x1b80
       worker_thread+0x8ff/0x1370
       kthread+0x32c/0x3f0
       ret_from_fork+0x27/0x50

-> #2 (slab_mutex){+.+.}:
       __lock_acquire+0x728/0x1200
       lock_acquire+0x269/0x5a0
       __mutex_lock+0x168/0x1730
       mutex_lock_nested+0x1b/0x20
       kmem_cache_create_usercopy+0x45/0x270
       kmem_cache_create+0x16/0x20
       ptlock_cache_init+0x24/0x2d
       start_kernel+0x40e/0x7e0
       x86_64_start_reservations+0x24/0x26
       x86_64_start_kernel+0xef/0xf6
       secondary_startup_64+0xb6/0xc0

-> #1 (memcg_cache_ids_sem){++++}:
       ptlock_cache_init+0x24/0x2d
       start_kernel+0x40e/0x7e0
       x86_64_start_reservations+0x24/0x26
       x86_64_start_kernel+0xef/0xf6
       secondary_startup_64+0xb6/0xc0

-> #0 (mem_hotplug_lock.rw_sem){++++}:
       validate_chain.isra.14+0x11af/0x3b50
       __lock_acquire+0x728/0x1200
       lock_acquire+0x269/0x5a0
       get_online_mems+0x3d/0x80
       show_slab_objects+0x16c/0x450
       total_objects_show+0x13/0x20
       slab_attr_show+0x1e/0x30
       sysfs_kf_seq_show+0x1d5/0x470
       kernfs_seq_show+0x1fa/0x2c0
       seq_read+0x3f7/0x1050
       kernfs_fop_read+0x126/0x650
       __vfs_read+0xeb/0xf20
       vfs_read+0x103/0x290
       ksys_read+0xfa/0x260
       __x64_sys_read+0x73/0xb0
       do_syscall_64+0x18f/0xd23
       entry_SYSCALL_64_after_hwframe+0x49/0xbe

other info that might help us debug this:

Chain exists of:
  mem_hotplug_lock.rw_sem --> slab_mutex --> kn->count#69

 Possible unsafe locking scenario:

       CPU0                    CPU1
       CPU0                    CPU1
       ----                    ----
  lock(kn->count#69);
                               lock(slab_mutex);
                               lock(kn->count#69);
  lock(mem_hotplug_lock.rw_sem);


3 locks held by read_all/7952:
 #0: 0000000005c4ddec (&p->lock){+.+.}, at: seq_read+0x6b/0x1050
 #1: 00000000c2f2e854 (&of->mutex){+.+.}, at: kernfs_seq_start+0x4f/0x170
 #2: 000000008804717f (kn->count#69){++++}, at: kernfs_seq_start+0x79/0x170