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BUG: KCSAN: data-race in pcpu_alloc / pcpu_balance_workfn

read-write to 0xffffffff86efc31c of 4 bytes by task 28 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+0x94c/0xa50 mm/percpu.c:2238
 process_one_work kernel/workqueue.c:2630 [inline]
 process_scheduled_works+0x5b8/0xa30 kernel/workqueue.c:2703
 worker_thread+0x525/0x730 kernel/workqueue.c:2784
 kthread+0x1d7/0x210 kernel/kthread.c:388
 ret_from_fork+0x48/0x60 arch/x86/kernel/process.c:147
 ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:242

read to 0xffffffff86efc31c of 4 bytes by task 16215 on cpu 0:
 pcpu_alloc+0x9bc/0x10b0 mm/percpu.c:1868
 __alloc_percpu_gfp+0x26/0x30 mm/percpu.c:1930
 ip_tunnel_init+0x59/0x270 net/ipv4/ip_tunnel.c:1249
 vti_tunnel_init+0xbe/0xd0 net/ipv4/ip_vti.c:436
 register_netdevice+0x1f0/0xd60 net/core/dev.c:10145
 __ip_tunnel_create+0x1c5/0x260 net/ipv4/ip_tunnel.c:267
 ip_tunnel_init_net+0x183/0x3c0 net/ipv4/ip_tunnel.c:1091
 vti_init_net+0x39/0xe0 net/ipv4/ip_vti.c:504
 ops_init+0x1fc/0x2c0 net/core/net_namespace.c:136
 setup_net+0x2cd/0x710 net/core/net_namespace.c:339
 copy_net_ns+0x384/0x520 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+0x3d3/0x720 kernel/fork.c:3433
 __do_sys_unshare kernel/fork.c:3504 [inline]
 __se_sys_unshare kernel/fork.c:3502 [inline]
 __x64_sys_unshare+0x1f/0x30 kernel/fork.c:3502
 do_syscall_x64 arch/x86/entry/common.c:51 [inline]
 do_syscall_64+0x44/0x110 arch/x86/entry/common.c:82
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

value changed: 0x00000001 -> 0x00000004

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 16215 Comm: syz-executor.0 Not tainted 6.7.0-rc2-syzkaller-00242-g090472ed9c92 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/10/2023
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