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Subjectlockdep warns: cpu_hotplug_lock.rw_sem --> slab_mutex --> kn->count#39
Hi,

I observed following warning from lockdep triggerd by 'slabinfo -d'
right after boot. There is nothing funky running in background, it's a
minimal rootfs created with buildroot.

Kernel version: v5.5.0-rc1
Kernel config for x86_64: defconfig + kvmconfig + lockdep + debug locks

# slabinfo -d
Acpi-Namespace not empty cannot disable sanity checks
Acpi-Namespace not empty cannot disable redzoning
Acpi-Operand not empty cannot disable sanity checks
Acpi-Operand not empty cannot disable redzoning
[ 5.033027]
[ 5.033173] ======================================================
[ 5.033712] WARNING: possible circular locking dependency detected
[ 5.034250] 5.5.0-rc1 #11 Not tainted
[ 5.034572] ------------------------------------------------------
[ 5.035108] slabinfo/136 is trying to acquire lock:
[ 5.035541] ffffffff8265eff0 (cpu_hotplug_lock.rw_sem){++++}, at: kmem_cache_shrink_all+0x9/0x20
[ 5.036313]
[ 5.036313] but task is already holding lock:
[ 5.036833] ffff88803cec63f8 (kn->count#39){++++}, at: kernfs_fop_write+0x9b/0x1b0
[ 5.037500]
[ 5.037500] which lock already depends on the new lock.
[ 5.037500]
[ 5.038208]
[ 5.038208] the existing dependency chain (in reverse order) is:
[ 5.038862]
[ 5.038862] -> #2 (kn->count#39){++++}:
[ 5.039329] __kernfs_remove+0x240/0x2e0
[ 5.039717] kernfs_remove_by_name_ns+0x3c/0x80
[ 5.040159] sysfs_slab_add+0x184/0x250
[ 5.040551] __kmem_cache_create+0x37c/0x410
[ 5.040969] kmem_cache_create_usercopy+0x150/0x270
[ 5.041442] kmem_cache_create+0xd/0x10
[ 5.041831] i915_global_scheduler_init+0x1e/0x7c
[ 5.042289] i915_globals_init+0xf/0x95
[ 5.042668] i915_init+0x8/0x5d
[ 5.042988] do_one_initcall+0x58/0x29f
[ 5.043375] kernel_init_freeable+0x1ae/0x238
[ 5.043804] kernel_init+0x5/0xf7
[ 5.044139] ret_from_fork+0x27/0x50
[ 5.044499]
[ 5.044499] -> #1 (slab_mutex){+.+.}:
[ 5.044950] __mutex_lock+0x86/0x900
[ 5.045310] kmem_cache_create_usercopy+0x32/0x270
[ 5.045768] kmem_cache_create+0xd/0x10
[ 5.046146] ptlock_cache_init+0x1b/0x23
[ 5.046534] start_kernel+0x21f/0x4f4
[ 5.046898] secondary_startup_64+0xb6/0xc0
[ 5.047309]
[ 5.047309] -> #0 (cpu_hotplug_lock.rw_sem){++++}:
[ 5.047849] __lock_acquire+0xe2f/0x19f0
[ 5.048233] lock_acquire+0x95/0x180
[ 5.048604] cpus_read_lock+0x26/0x90
[ 5.048970] kmem_cache_shrink_all+0x9/0x20
[ 5.049386] shrink_store+0xe/0x20
[ 5.049730] slab_attr_store+0x1b/0x30
[ 5.050103] kernfs_fop_write+0xca/0x1b0
[ 5.050494] vfs_write+0xb4/0x1a0
[ 5.050830] ksys_write+0x63/0xe0
[ 5.051165] do_syscall_64+0x4b/0x1e0
[ 5.051533] entry_SYSCALL_64_after_hwframe+0x49/0xbe
[ 5.052012]
[ 5.052012] other info that might help us debug this:
[ 5.052012]
[ 5.052707] Chain exists of:
[ 5.052707] cpu_hotplug_lock.rw_sem --> slab_mutex --> kn->count#39
[ 5.052707]
[ 5.053629] Possible unsafe locking scenario:
[ 5.053629]
[ 5.054137] CPU0 CPU1
[ 5.054529] ---- ----
[ 5.054922] lock(kn->count#39);
[ 5.055211] lock(slab_mutex);
[ 5.055695] lock(kn->count#39);
[ 5.056195] lock(cpu_hotplug_lock.rw_sem);
[ 5.056580]
[ 5.056580] *** DEADLOCK ***
[ 5.056580]
[ 5.057087] 3 locks held by slabinfo/136:
[ 5.057439] #0: ffff88803c151480 (sb_writers#7){.+.+}, at: vfs_write+0x13b/0x1a0
[ 5.058078] #1: ffff88803c1a7890 (&of->mutex){+.+.}, at: kernfs_fop_write+0x93/0x1b0
[ 5.058745] #2: ffff88803cec63f8 (kn->count#39){++++}, at: kernfs_fop_write+0x9b/0x1b0
[ 5.059426]
[ 5.059426] stack backtrace:
[ 5.059803] CPU: 0 PID: 136 Comm: slabinfo Not tainted 5.5.0-rc1 #11
[ 5.060349] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-59-gc9ba527-rebuilt.opensuse.org 04/01/2014
[ 5.061320] Call Trace:
[ 5.061538] dump_stack+0x71/0xa0
[ 5.061829] check_noncircular+0x176/0x190
[ 5.062193] __lock_acquire+0xe2f/0x19f0
[ 5.062542] lock_acquire+0x95/0x180
[ 5.062856] ? kmem_cache_shrink_all+0x9/0x20
[ 5.063236] cpus_read_lock+0x26/0x90
[ 5.063555] ? kmem_cache_shrink_all+0x9/0x20
[ 5.063931] kmem_cache_shrink_all+0x9/0x20
[ 5.064296] shrink_store+0xe/0x20
[ 5.064604] slab_attr_store+0x1b/0x30
[ 5.064931] kernfs_fop_write+0xca/0x1b0
[ 5.065273] vfs_write+0xb4/0x1a0
[ 5.065564] ksys_write+0x63/0xe0
[ 5.065855] do_syscall_64+0x4b/0x1e0
[ 5.066175] entry_SYSCALL_64_after_hwframe+0x49/0xbe
[ 5.066610] RIP: 0033:0x45c283
[ 5.066877] Code: ff ff f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb bb 0f 1f 80 00 00 00 00 64 8b 04 25 18 00 00 00 85 c0 75 14 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 55 c3 0f 1f 40 00 48 83 ec 28 48 89 54 24 18
[ 5.068456] RSP: 002b:00007ffd3d486908 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[ 5.069106] RAX: ffffffffffffffda RBX: 0000000000000002 RCX: 000000000045c283
[ 5.069728] RDX: 0000000000000002 RSI: 0000000002342360 RDI: 0000000000000003
[ 5.070347] RBP: 0000000002342360 R08: 0000000000000000 R09: 0000000000000002
[ 5.070964] R10: 0000000000000001 R11: 0000000000000246 R12: 0000000000000002
[ 5.071574] R13: 000000000234a3a0 R14: 0000000000000002 R15: 00000000004c6ec0
Cannot write to Acpi-Parse/sanity

Thanks,
Daniel

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