Spurious lockdep splat in v4.15-rc9

[Tejun Heo]

Hello, Peter, Ingo.

I get the below lockdep warning if I try to write a config into cgroup
cpu.max file.  It's warning about A-A deadlock, but it's obviously
spurious - the system doesn't lock up and the warning is about two
get_online_cpus() calls nesting.

Thanks.

[   79.106704]
[   79.106886] ============================================
[   79.107319] WARNING: possible recursive locking detected
[   79.107741] 4.15.0-rc9-work+ #61 Not tainted
[   79.108080] --------------------------------------------
[   79.108505] bash/2133 is trying to acquire lock:
[   79.108872]  (cpu_hotplug_lock.rw_sem){++++}, at: [<00000000b3203afd>] static_key_slow_inc+0xe/0xa0
[   79.109593]
[   79.109593] but task is already holding lock:
[   79.110058]  (cpu_hotplug_lock.rw_sem){++++}, at: [<00000000748e6cec>] tg_set_cfs_bandwidth+0x51/0x330
[   79.110801]
[   79.110801] other info that might help us debug this:
[   79.111322]  Possible unsafe locking scenario:
[   79.111322]
[   79.111792]        CPU0
[   79.111992]        ----
[   79.112197]   lock(cpu_hotplug_lock.rw_sem);
[   79.112537]   lock(cpu_hotplug_lock.rw_sem);
[   79.112880]
[   79.112880]  *** DEADLOCK ***
[   79.112880]
[   79.113355]  May be due to missing lock nesting notation
[   79.113355]
[   79.113893] 5 locks held by bash/2133:
[   79.114199]  #0:  (sb_writers#7){.+.+}, at: [<00000000259a9362>] vfs_write+0x18a/0x1c0
[   79.114830]  #1:  (&of->mutex){+.+.}, at: [<00000000b1a2a028>] kernfs_fop_write+0xde/0x1a0
[   79.115492]  #2:  (kn->count#182){.+.+}, at: [<000000008f74a9a4>] kernfs_fop_write+0xe6/0x1a0
[   79.116182]  #3:  (cpu_hotplug_lock.rw_sem){++++}, at: [<00000000748e6cec>] tg_set_cfs_bandwidth+0x51/0x330
[   79.116956]  #4:  (cfs_constraints_mutex){+.+.}, at: [<000000007a63f0e9>] tg_set_cfs_bandwidth+0x5f/0x330
[   79.117717]
[   79.117717] stack backtrace:
[   79.118072] CPU: 13 PID: 2133 Comm: bash Not tainted 4.15.0-rc9-work+ #61
[   79.118616] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-3.el7_4.1 04/01/2014
[   79.119331] Call Trace:
[   79.119539]  dump_stack+0x5e/0x8f
[   79.119809]  __lock_acquire+0x150e/0x15f0
[   79.120136]  ? tg_set_cfs_bandwidth+0x5f/0x330
[   79.120497]  ? __mutex_lock+0x204/0x930
[   79.120805]  ? tg_set_cfs_bandwidth+0x5f/0x330
[   79.121164]  lock_acquire+0xb0/0x200
[   79.121454]  ? static_key_slow_inc+0xe/0xa0
[   79.121798]  cpus_read_lock+0x43/0xb0
[   79.122095]  ? static_key_slow_inc+0xe/0xa0
[   79.122438]  static_key_slow_inc+0xe/0xa0
[   79.122763]  tg_set_cfs_bandwidth+0x30e/0x330
[   79.123112]  ? tg_set_cfs_bandwidth+0xaa/0x330
[   79.123473]  cpu_max_write+0xb8/0x100
[   79.123773]  cgroup_file_write+0x69/0x200
[   79.124100]  kernfs_fop_write+0x10e/0x1a0
[   79.124430]  __vfs_write+0x23/0x130
[   79.124721]  ? rcu_read_lock_sched_held+0x96/0xa0
[   79.125100]  ? rcu_sync_lockdep_assert+0x2a/0x50
[   79.125475]  ? __sb_start_write+0x194/0x230
[   79.125812]  ? vfs_write+0x18a/0x1c0
[   79.126109]  ? __close_fd+0x66/0xd0
[   79.126392]  vfs_write+0xbf/0x1c0
[   79.126660]  SyS_write+0x45/0xa0
[   79.126928]  entry_SYSCALL_64_fastpath+0x18/0x85
[   79.127306] RIP: 0033:0x7f3b040e21f0

-- 
tejun