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BUG: KCSAN: data-race in find_alive_thread / get_signal

write to 0xffff88810d6ee0a4 of 4 bytes by task 7754 on cpu 0:
 get_signal+0x7a2/0x1290 kernel/signal.c:2714
 do_signal+0x2b/0x670 arch/x86/kernel/signal.c:813
 exit_to_usermode_loop+0x24a/0x2c0 arch/x86/entry/common.c:160
 prepare_exit_to_usermode arch/x86/entry/common.c:195 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:278 [inline]
 do_syscall_64+0x36e/0x390 arch/x86/entry/common.c:304
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88810d6ee0a4 of 4 bytes by task 7747 on cpu 1:
 find_alive_thread+0xd5/0x120 kernel/exit.c:495
 find_new_reaper kernel/exit.c:544 [inline]
 forget_original_parent kernel/exit.c:619 [inline]
 exit_notify kernel/exit.c:652 [inline]
 do_exit+0x7f5/0x1880 kernel/exit.c:822
 do_group_exit+0xae/0x1a0 kernel/exit.c:899
 get_signal+0x2a7/0x1290 kernel/signal.c:2734
 do_signal+0x2b/0x670 arch/x86/kernel/signal.c:813
 exit_to_usermode_loop+0x24a/0x2c0 arch/x86/entry/common.c:160
 prepare_exit_to_usermode arch/x86/entry/common.c:195 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:278 [inline]
 do_syscall_64+0x36e/0x390 arch/x86/entry/common.c:304
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 7747 Comm: syz-executor.4 Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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