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BUG: KCSAN: data-race in pcpu_alloc / pcpu_balance_workfn

read-write to 0xffffffff86f595d4 of 4 bytes by task 3157 on cpu 1:
 pcpu_update_empty_pages mm/percpu.c:602 [inline]
 pcpu_chunk_populated mm/percpu.c:1531 [inline]
 pcpu_balance_populated mm/percpu.c:2088 [inline]
 pcpu_balance_workfn+0x96f/0xa80 mm/percpu.c:2238
 process_one_work+0x3e6/0x750 kernel/workqueue.c:2405
 worker_thread+0x5f2/0xa10 kernel/workqueue.c:2552
 kthread+0x1d7/0x210 kernel/kthread.c:379
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:308

read to 0xffffffff86f595d4 of 4 bytes by task 15369 on cpu 0:
 pcpu_alloc+0xa59/0x1280 mm/percpu.c:1870
 __alloc_percpu_gfp+0x26/0x30 mm/percpu.c:1930
 vti6_dev_init_gen net/ipv6/ip6_vti.c:934 [inline]
 vti6_dev_init+0x6d/0x1f0 net/ipv6/ip6_vti.c:948
 register_netdevice+0x217/0xcd0 net/core/dev.c:10015
 register_netdev+0x3b/0x50 net/core/dev.c:10213
 vti6_init_net+0x1e5/0x250 net/ipv6/ip6_vti.c:1162
 ops_init+0x1fc/0x2c0 net/core/net_namespace.c:136
 setup_net+0x2b5/0x700 net/core/net_namespace.c:339
 copy_net_ns+0x36c/0x500 net/core/net_namespace.c:491
 create_new_namespaces+0x228/0x430 kernel/nsproxy.c:110
 unshare_nsproxy_namespaces+0xe4/0x120 kernel/nsproxy.c:228
 ksys_unshare+0x38d/0x6d0 kernel/fork.c:3441
 __do_sys_unshare kernel/fork.c:3512 [inline]
 __se_sys_unshare kernel/fork.c:3510 [inline]
 __x64_sys_unshare+0x1f/0x30 kernel/fork.c:3510
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

value changed: 0x00000001 -> 0x00000004

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 15369 Comm: syz-executor.2 Not tainted 6.4.0-rc5-syzkaller-00016-ga4d7d7011219 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/25/2023
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