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BUG: KCSAN: data-race in get_cpu_idle_time_us / tick_nohz_idle_exit

read-write to 0xffff88813bc1e2bc of 1 bytes by task 0 on cpu 0:
 tick_nohz_idle_exit+0x8b/0x210 kernel/time/tick-sched.c:1199
 do_idle+0x1f6/0x230 kernel/sched/idle.c:286
 cpu_startup_entry+0x15/0x20 kernel/sched/idle.c:369
 rest_init+0xd7/0xe0 init/main.c:711
 arch_call_rest_init+0xa/0xb
 start_kernel+0x5a0/0x625 init/main.c:1051
 secondary_startup_64_no_verify+0xa6/0xab

read to 0xffff88813bc1e2bc of 1 bytes by task 18930 on cpu 1:
 get_cpu_idle_time_us+0x6b/0x1f0 kernel/time/tick-sched.c:599
 get_idle_time fs/proc/stat.c:53 [inline]
 show_stat+0x26f/0xdc0 fs/proc/stat.c:131
 seq_read+0x2de/0x890 fs/seq_file.c:208
 pde_read fs/proc/inode.c:321 [inline]
 proc_reg_read+0xe1/0x1b0 fs/proc/inode.c:331
 do_loop_readv_writev fs/read_write.c:761 [inline]
 do_iter_read+0x3b1/0x4b0 fs/read_write.c:803
 vfs_readv fs/read_write.c:921 [inline]
 do_preadv+0x139/0x210 fs/read_write.c:1013
 __do_sys_preadv fs/read_write.c:1063 [inline]
 __se_sys_preadv fs/read_write.c:1058 [inline]
 __x64_sys_preadv+0x54/0x60 fs/read_write.c:1058
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 18930 Comm: syz-executor.3 Not tainted 5.9.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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