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SubjectLockdep splat in -next
Hello, Yixian Liu,

The following -next commit results in a lockdep splat:

591f762b2750 ("RDMA/hns: Remove the condition of light load for posting DWQE")

The repeat-by is as follows:

tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --trust-make --duration 1 --configs TASKS01

The resulting splat is as shown below. This appears to have been either
fixed or obscured by a later commit, but it does affect bisectability.
Which I found out about the hard way. ;-)

Thanx, Paul

======================================================
WARNING: possible circular locking dependency detected
5.13.0-rc1+ #2218 Not tainted
------------------------------------------------------
rcu_torture_sta/66 is trying to acquire lock:
ffffffffa0063c90 (cpu_hotplug_lock){++++}-{0:0}, at: static_key_enable+0x9/0x20

but task is already holding lock:
ffffffffa01754e8 (slab_mutex){+.+.}-{3:3}, at: kmem_cache_create_usercopy+0x2d/0x250

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #1 (slab_mutex){+.+.}-{3:3}:
__mutex_lock+0x99/0x950
slub_cpu_dead+0x15/0xf0
cpuhp_invoke_callback+0x181/0x850
cpuhp_invoke_callback_range+0x3b/0x80
_cpu_down+0xdf/0x2a0
cpu_down+0x2c/0x50
device_offline+0x82/0xb0
remove_cpu+0x1a/0x30
torture_offline+0x80/0x140
torture_onoff+0x147/0x260
kthread+0x123/0x160
ret_from_fork+0x22/0x30

-> #0 (cpu_hotplug_lock){++++}-{0:0}:
__lock_acquire+0x12e6/0x27c0
lock_acquire+0xc8/0x3a0
cpus_read_lock+0x26/0xb0
static_key_enable+0x9/0x20
__kmem_cache_create+0x39e/0x440
kmem_cache_create_usercopy+0x146/0x250
kmem_cache_create+0xd/0x10
rcu_torture_stats+0x79/0x280
kthread+0x123/0x160
ret_from_fork+0x22/0x30

other info that might help us debug this:

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(slab_mutex);
lock(cpu_hotplug_lock);
lock(slab_mutex);
lock(cpu_hotplug_lock);

*** DEADLOCK ***

1 lock held by rcu_torture_sta/66:
#0: ffffffffa01754e8 (slab_mutex){+.+.}-{3:3}, at: kmem_cache_create_usercopy+0x2d/0x250

stack backtrace:
CPU: 1 PID: 66 Comm: rcu_torture_sta Not tainted 5.13.0-rc1+ #2218
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-1ubuntu1.1 04/01/2014
Call Trace:
dump_stack+0x6d/0x89
check_noncircular+0xfe/0x110
? rcu_read_lock_sched_held+0x4d/0x80
? __alloc_pages+0x329/0x360
__lock_acquire+0x12e6/0x27c0
lock_acquire+0xc8/0x3a0
? static_key_enable+0x9/0x20
cpus_read_lock+0x26/0xb0
? static_key_enable+0x9/0x20
static_key_enable+0x9/0x20
__kmem_cache_create+0x39e/0x440
kmem_cache_create_usercopy+0x146/0x250
? rcu_torture_stats_print+0xd0/0xd0
kmem_cache_create+0xd/0x10
rcu_torture_stats+0x79/0x280
kthread+0x123/0x160
? kthread_park+0x80/0x80
ret_from_fork+0x22/0x30

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