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BUG: KCSAN: data-race in pcpu_alloc_noprof / pcpu_balance_workfn

read-write to 0xffffffff88be03ac of 4 bytes by task 3391 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:2062 [inline]
 pcpu_balance_workfn+0x94e/0xa60 mm/percpu.c:2212
 process_one_work kernel/workqueue.c:3229 [inline]
 process_scheduled_works+0x483/0x9a0 kernel/workqueue.c:3310
 worker_thread+0x51d/0x6f0 kernel/workqueue.c:3391
 kthread+0x1d1/0x210 kernel/kthread.c:389
 ret_from_fork+0x4b/0x60 arch/x86/kernel/process.c:147
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

read to 0xffffffff88be03ac of 4 bytes by task 3701 on cpu 0:
 pcpu_alloc_noprof+0x9b6/0x10a0 mm/percpu.c:1894
 fib_nh_common_init+0x4a/0x2b0 net/ipv4/fib_semantics.c:590
 fib6_nh_init+0xea5/0x15d0 net/ipv6/route.c:3622
 ip6_route_info_create+0x7b2/0xba0 net/ipv6/route.c:3809
 ip6_route_add+0x26/0x120 net/ipv6/route.c:3853
 addrconf_add_mroute net/ipv6/addrconf.c:2550 [inline]
 addrconf_add_dev+0x1c6/0x240 net/ipv6/addrconf.c:2568
 addrconf_dev_config net/ipv6/addrconf.c:3484 [inline]
 addrconf_init_auto_addrs+0x497/0x6b0 net/ipv6/addrconf.c:3572
 addrconf_notify+0x7ae/0x950 net/ipv6/addrconf.c:3745
 notifier_call_chain kernel/notifier.c:93 [inline]
 raw_notifier_call_chain+0x6f/0x1d0 kernel/notifier.c:461
 call_netdevice_notifiers_info+0xae/0x100 net/core/dev.c:1996
 __dev_notify_flags+0xff/0x1a0
 dev_change_flags+0xab/0xd0 net/core/dev.c:8922
 do_setlink+0x844/0x2490 net/core/rtnetlink.c:2929
 __rtnl_newlink net/core/rtnetlink.c:3725 [inline]
 rtnl_newlink+0x11a3/0x1690 net/core/rtnetlink.c:3772
 rtnetlink_rcv_msg+0x6aa/0x710 net/core/rtnetlink.c:6675
 netlink_rcv_skb+0x12c/0x230 net/netlink/af_netlink.c:2551
 rtnetlink_rcv+0x1c/0x30 net/core/rtnetlink.c:6693
 netlink_unicast_kernel net/netlink/af_netlink.c:1331 [inline]
 netlink_unicast+0x599/0x670 net/netlink/af_netlink.c:1357
 netlink_sendmsg+0x5cc/0x6e0 net/netlink/af_netlink.c:1901
 sock_sendmsg_nosec net/socket.c:729 [inline]
 __sock_sendmsg+0x140/0x180 net/socket.c:744
 __sys_sendto+0x1d6/0x260 net/socket.c:2214
 __do_sys_sendto net/socket.c:2226 [inline]
 __se_sys_sendto net/socket.c:2222 [inline]
 __x64_sys_sendto+0x78/0x90 net/socket.c:2222
 x64_sys_call+0x2959/0x2d60 arch/x86/include/generated/asm/syscalls_64.h:45
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x00000001 -> 0x00000004

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 3701 Comm: syz-executor Not tainted 6.12.0-rc6-syzkaller-00192-gf1dce1f09380 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
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syz-executor (3701) used greatest stack depth: 9200 bytes left