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BUG: KCSAN: data-race in pcpu_alloc / pcpu_balance_workfn

read-write to 0xffffffff870d7b74 of 4 bytes by task 34 on cpu 0:
 pcpu_update_empty_pages mm/percpu.c:606 [inline]
 pcpu_chunk_populated mm/percpu.c:1534 [inline]
 pcpu_balance_populated mm/percpu.c:2095 [inline]
 pcpu_balance_workfn+0xa58/0xb90 mm/percpu.c:2246
 process_one_work+0x3d3/0x720 kernel/workqueue.c:2289
 worker_thread+0x618/0xa70 kernel/workqueue.c:2436
 kthread+0x1a9/0x1e0 kernel/kthread.c:376
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:306

read to 0xffffffff870d7b74 of 4 bytes by task 19713 on cpu 1:
 pcpu_alloc+0x91d/0xf60 mm/percpu.c:1877
 __alloc_percpu_gfp+0x22/0x30 mm/percpu.c:1937
 __percpu_counter_init+0x43/0x130 lib/percpu_counter.c:149
 dst_entries_init include/net/dst_ops.h:65 [inline]
 xfrm6_net_init+0x45/0x140 net/ipv6/xfrm6_policy.c:253
 ops_init+0x215/0x2d0 net/core/net_namespace.c:135
 setup_net+0x2cb/0x810 net/core/net_namespace.c:332
 copy_net_ns+0x2a2/0x440 net/core/net_namespace.c:478
 create_new_namespaces+0x22c/0x450 kernel/nsproxy.c:110
 unshare_nsproxy_namespaces+0xe2/0x120 kernel/nsproxy.c:226
 ksys_unshare+0x38c/0x6e0 kernel/fork.c:3188
 __do_sys_unshare kernel/fork.c:3259 [inline]
 __se_sys_unshare kernel/fork.c:3257 [inline]
 __x64_sys_unshare+0x1b/0x20 kernel/fork.c:3257
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x2b/0x70 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

value changed: 0x00000001 -> 0xfffffffc

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 19713 Comm: syz-executor.3 Not tainted 6.1.0-rc6-syzkaller-dirty #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/26/2022
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