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BUG: KCSAN: data-race in get_cpu_idle_time_us / tick_nohz_idle_exit

write to 0xffff88812c01f17c of 1 bytes by task 0 on cpu 0:
 tick_nohz_idle_exit+0xcb/0x290 kernel/time/tick-sched.c:1199
 do_idle+0x280/0x2c0 kernel/sched/idle.c:286
 cpu_startup_entry+0x15/0x20 kernel/sched/idle.c:369
 rest_init+0xd8/0xe0 init/main.c:708
 arch_call_rest_init+0xa/0xb
 start_kernel+0x608/0x6d0 init/main.c:1048
 secondary_startup_64+0xa4/0xb0 arch/x86/kernel/head_64.S:243

read to 0xffff88812c01f17c of 1 bytes by task 19811 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+0x7ff/0xd10 fs/proc/stat.c:172
 seq_read+0x2ee/0x900 fs/seq_file.c:208
 pde_read fs/proc/inode.c:306 [inline]
 proc_reg_read+0xde/0x1a0 fs/proc/inode.c:316
 do_loop_readv_writev fs/read_write.c:734 [inline]
 do_iter_read+0x3d2/0x4b0 fs/read_write.c:955
 vfs_readv fs/read_write.c:1073 [inline]
 do_preadv+0x13b/0x230 fs/read_write.c:1165
 __do_sys_preadv fs/read_write.c:1215 [inline]
 __se_sys_preadv fs/read_write.c:1210 [inline]
 __x64_sys_preadv+0x54/0x60 fs/read_write.c:1210
 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: 19811 Comm: syz-executor.0 Not tainted 5.9.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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