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BUG: KCSAN: data-race in pcpu_alloc / pcpu_balance_workfn

write to 0xffffffff86ddd764 of 4 bytes by task 25 on cpu 1:
 pcpu_update_empty_pages mm/percpu.c:606 [inline]
 pcpu_chunk_populated mm/percpu.c:1534 [inline]
 pcpu_balance_populated mm/percpu.c:2097 [inline]
 pcpu_balance_workfn+0xb04/0xca0 mm/percpu.c:2248
 process_one_work+0x3f6/0x960 kernel/workqueue.c:2307
 worker_thread+0x616/0xa70 kernel/workqueue.c:2454
 kthread+0x2c7/0x2e0 kernel/kthread.c:327
 ret_from_fork+0x1f/0x30

read to 0xffffffff86ddd764 of 4 bytes by task 14852 on cpu 0:
 pcpu_alloc+0x9c0/0xf80 mm/percpu.c:1878
 __alloc_percpu+0x20/0x30 mm/percpu.c:1950
 ipv6_init_mibs net/ipv6/af_inet6.c:895 [inline]
 inet6_net_init+0x25a/0x490 net/ipv6/af_inet6.c:968
 ops_init+0x1e7/0x230 net/core/net_namespace.c:140
 setup_net+0x1fb/0x740 net/core/net_namespace.c:328
 copy_net_ns+0x2a9/0x450 net/core/net_namespace.c:472
 create_new_namespaces+0x231/0x560 kernel/nsproxy.c:110
 unshare_nsproxy_namespaces+0xe2/0x120 kernel/nsproxy.c:226
 ksys_unshare+0x376/0x6f0 kernel/fork.c:3049
 __do_sys_unshare kernel/fork.c:3120 [inline]
 __se_sys_unshare kernel/fork.c:3118 [inline]
 __x64_sys_unshare+0x1b/0x20 kernel/fork.c:3118
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x00000001 -> 0x00000004

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 14852 Comm: syz-executor.4 Not tainted 5.16.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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