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BUG: KCSAN: data-race in complete_signal / do_group_exit

write to 0xffff8881003a6db4 of 4 bytes by task 1749 on cpu 0:
 complete_signal+0x4a5/0x600 kernel/signal.c:1084
 __send_signal+0x680/0x760 kernel/signal.c:1221
 send_signal+0x281/0x390 kernel/signal.c:1284
 do_send_sig_info kernel/signal.c:1333 [inline]
 group_send_sig_info+0xe8/0x140 kernel/signal.c:1456
 kill_pid_info kernel/signal.c:1490 [inline]
 kill_proc_info kernel/signal.c:1507 [inline]
 kill_something_info kernel/signal.c:1604 [inline]
 __do_sys_kill kernel/signal.c:3745 [inline]
 __se_sys_kill+0xeb/0x400 kernel/signal.c:3739
 __x64_sys_kill+0x2d/0x40 kernel/signal.c:3739
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff8881003a6db4 of 4 bytes by task 32033 on cpu 1:
 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

value changed: 0x00000000 -> 0x00000004

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 32033 Comm: syz-executor.5 Not tainted 5.13.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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