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BUG: KCSAN: data-race in do_exit / get_signal

read-write to 0xffff88806770f064 of 4 bytes by task 1807 on cpu 1:
 get_signal+0xee8/0x1510 kernel/signal.c:2738
 arch_do_signal+0x25/0x260 arch/x86/kernel/signal.c:811
 exit_to_user_mode_loop kernel/entry/common.c:161 [inline]
 exit_to_user_mode_prepare+0xde/0x170 kernel/entry/common.c:191
 syscall_exit_to_user_mode+0x16/0x30 kernel/entry/common.c:266
 do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88806770f064 of 4 bytes by task 1810 on cpu 0:
 find_alive_thread kernel/exit.c:496 [inline]
 find_new_reaper kernel/exit.c:545 [inline]
 forget_original_parent kernel/exit.c:620 [inline]
 exit_notify kernel/exit.c:653 [inline]
 do_exit+0x53c/0x1630 kernel/exit.c:829
 do_group_exit+0x16f/0x170 kernel/exit.c:906
 __do_sys_exit_group+0xb/0x10 kernel/exit.c:917
 __se_sys_exit_group+0x5/0x10 kernel/exit.c:915
 __x64_sys_exit_group+0x16/0x20 kernel/exit.c:915
 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: 1810 Comm: syz-executor.4 Not tainted 5.10.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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