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BUG: KCSAN: data-race in pcpu_alloc / pcpu_balance_workfn

write to 0xffffffff8700379c of 4 bytes by task 2011 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:2094 [inline]
 pcpu_balance_workfn+0xb63/0xd10 mm/percpu.c:2245
 process_one_work+0x3e3/0x970 kernel/workqueue.c:2289
 worker_thread+0x638/0xaa0 kernel/workqueue.c:2436
 kthread+0x1bf/0x1e0 kernel/kthread.c:376
 ret_from_fork+0x1f/0x30

read to 0xffffffff8700379c of 4 bytes by task 3360 on cpu 0:
 pcpu_alloc+0x99c/0xf50 mm/percpu.c:1877
 __alloc_percpu_gfp+0x22/0x30 mm/percpu.c:1936
 ip_tunnel_init+0x52/0x220 net/ipv4/ip_tunnel.c:1228
 gre_tap_init+0x21e/0x230 net/ipv4/ip_gre.c:1279
 register_netdevice+0x404/0x10d0 net/core/dev.c:9891
 __ip_tunnel_create+0x1c5/0x260 net/ipv4/ip_tunnel.c:267
 ip_tunnel_init_net+0x19a/0x3e0 net/ipv4/ip_tunnel.c:1070
 ipgre_tap_init_net+0x31/0x40 net/ipv4/ip_gre.c:1686
 ops_init+0x1e7/0x230 net/core/net_namespace.c:134
 setup_net+0x2cb/0x810 net/core/net_namespace.c:325
 copy_net_ns+0x2a9/0x450 net/core/net_namespace.c:471
 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:3132
 __do_sys_unshare kernel/fork.c:3203 [inline]
 __se_sys_unshare kernel/fork.c:3201 [inline]
 __x64_sys_unshare+0x1b/0x20 kernel/fork.c:3201
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x2b/0x50 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: 3360 Comm: syz-executor.5 Not tainted 5.17.0-syzkaller-12882-g1930a6e739c4-dirty #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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