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BUG: KCSAN: data-race in find_alive_thread / get_signal

write to 0xffff88808ebc1064 of 4 bytes by task 13541 on cpu 1:
 get_signal+0x7a2/0x1290 kernel/signal.c:2719
 do_signal+0x2b/0x840 arch/x86/kernel/signal.c:784
 exit_to_usermode_loop+0x24a/0x2c0 arch/x86/entry/common.c:161
 prepare_exit_to_usermode arch/x86/entry/common.c:196 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:279 [inline]
 do_syscall_64+0x38b/0x3b0 arch/x86/entry/common.c:305
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88808ebc1064 of 4 bytes by task 13543 on cpu 0:
 find_alive_thread+0xd5/0x120 kernel/exit.c:489
 find_new_reaper kernel/exit.c:538 [inline]
 forget_original_parent+0xef/0x980 kernel/exit.c:613
 exit_notify kernel/exit.c:646 [inline]
 do_exit+0x75c/0x1150 kernel/exit.c:816
 do_group_exit+0xae/0x1a0 kernel/exit.c:893
 get_signal+0x2a7/0x1290 kernel/signal.c:2739
 do_signal+0x2b/0x840 arch/x86/kernel/signal.c:784
 exit_to_usermode_loop+0x24a/0x2c0 arch/x86/entry/common.c:161
 prepare_exit_to_usermode arch/x86/entry/common.c:196 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:279 [inline]
 do_syscall_64+0x38b/0x3b0 arch/x86/entry/common.c:305
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 13543 Comm: syz-executor.4 Not tainted 5.7.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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