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BUG: KCSAN: data-race in copy_process / show_stat

write to 0xffffffff894cb408 of 8 bytes by task 30 on cpu 0:
 copy_process+0x295e/0x2dc0 kernel/fork.c:2270
 _do_fork+0x14a/0x6c0 kernel/fork.c:2428
 kernel_thread+0x85/0xb0 kernel/fork.c:2480
 call_usermodehelper_exec_work+0x3d/0x150 kernel/umh.c:163
 process_one_work+0x3e1/0x9a0 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffffffff894cb408 of 8 bytes by task 13916 on cpu 1:
 show_stat+0xba5/0xd10 fs/proc/stat.c:204
 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: 13916 Comm: syz-executor.2 Not tainted 5.9.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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