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BUG: KCSAN: data-race in do_exit / get_signal

write to 0xffff88809b1e5024 of 4 bytes by task 23354 on cpu 1:
 get_signal+0xdd5/0x1650 kernel/signal.c:2737
 arch_do_signal+0x25/0x270 arch/x86/kernel/signal.c:811
 exit_to_user_mode_loop kernel/entry/common.c:136 [inline]
 exit_to_user_mode_prepare+0x124/0x210 kernel/entry/common.c:167
 syscall_exit_to_user_mode+0x16/0x30 kernel/entry/common.c:242
 do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88809b1e5024 of 4 bytes by task 23357 on cpu 0:
 find_alive_thread kernel/exit.c:493 [inline]
 find_new_reaper kernel/exit.c:542 [inline]
 forget_original_parent kernel/exit.c:617 [inline]
 exit_notify kernel/exit.c:650 [inline]
 do_exit+0x7e0/0x16c0 kernel/exit.c:826
 do_group_exit+0x17a/0x180 kernel/exit.c:903
 __do_sys_exit_group+0xb/0x10 kernel/exit.c:914
 __se_sys_exit_group+0x5/0x10 kernel/exit.c:912
 __x64_sys_exit_group+0x16/0x20 kernel/exit.c:912
 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: 0 PID: 23357 Comm: syz-executor.5 Not tainted 5.9.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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