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BUG: KCSAN: data-race in free_pid / zap_pid_ns_processes

write to 0xffff888120cacd30 of 4 bytes by task 12017 on cpu 1:
 free_pid+0xfd/0x1c0 kernel/pid.c:134
 __change_pid+0x169/0x1e0 kernel/pid.c:305
 detach_pid+0x29/0x40 kernel/pid.c:310
 __unhash_process kernel/exit.c:78 [inline]
 __exit_signal kernel/exit.c:157 [inline]
 release_task+0x9f9/0xc60 kernel/exit.c:207
 exit_notify kernel/exit.c:687 [inline]
 do_exit+0xdae/0x18f0 kernel/exit.c:838
 do_group_exit+0xb4/0x1c0 kernel/exit.c:921
 get_signal+0x2a2/0x1320 kernel/signal.c:2734
 do_signal+0x3b/0xbf0 arch/x86/kernel/signal.c:815
 exit_to_usermode_loop+0x250/0x2c0 arch/x86/entry/common.c:159
 prepare_exit_to_usermode arch/x86/entry/common.c:194 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:274 [inline]
 do_syscall_64+0x353/0x370 arch/x86/entry/common.c:300
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888120cacd30 of 4 bytes by task 28046 on cpu 0:
 zap_pid_ns_processes+0x259/0x310 kernel/pid_namespace.c:253
 find_child_reaper kernel/exit.c:530 [inline]
 forget_original_parent kernel/exit.c:619 [inline]
 exit_notify kernel/exit.c:656 [inline]
 do_exit+0x16f1/0x18f0 kernel/exit.c:838
 do_group_exit+0xb4/0x1c0 kernel/exit.c:921
 __do_sys_exit_group kernel/exit.c:932 [inline]
 __se_sys_exit_group kernel/exit.c:930 [inline]
 __x64_sys_exit_group+0x2e/0x30 kernel/exit.c:930
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 28046 Comm: syz-executor.4 Not tainted 5.4.0-rc7+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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