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BUG: KCSAN: data-race in do_group_exit / do_group_exit

write to 0xffff88814c26d234 of 4 bytes by task 18541 on cpu 1:
 do_group_exit+0x136/0x1a0 kernel/exit.c:917
 __do_sys_exit_group+0xb/0x10 kernel/exit.c:934
 __se_sys_exit_group+0x5/0x10 kernel/exit.c:932
 __x64_sys_exit_group+0x16/0x20 kernel/exit.c:932
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88814c26d234 of 4 bytes by task 18515 on cpu 0:
 signal_group_exit include/linux/sched/signal.h:269 [inline]
 do_group_exit+0x51/0x1a0 kernel/exit.c:906
 __do_sys_exit_group+0xb/0x10 kernel/exit.c:934
 __se_sys_exit_group+0x5/0x10 kernel/exit.c:932
 __x64_sys_exit_group+0x16/0x20 kernel/exit.c:932
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 18515 Comm: syz-executor.1 Not tainted 5.13.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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