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BUG: KCSAN: data-race in complete_signal / do_group_exit

write to 0xffff88807ce1e0b4 of 4 bytes by task 24385 on cpu 1:
 complete_signal+0x4a5/0x600 kernel/signal.c:1044
 __send_signal+0x680/0x7d0 kernel/signal.c:1180
 send_signal+0x398/0x3e0 kernel/signal.c:1241
 do_send_sig_info+0x9b/0xe0 kernel/signal.c:1290
 send_sig_info kernel/signal.c:1605 [inline]
 send_sig+0x4f/0x60 kernel/signal.c:1615
 wait_for_space fs/splice.c:1071 [inline]
 do_splice+0x104c/0x10c0 fs/splice.c:1171
 __do_sys_splice fs/splice.c:1419 [inline]
 __se_sys_splice fs/splice.c:1401 [inline]
 __x64_sys_splice+0xf2/0x190 fs/splice.c:1401
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88807ce1e0b4 of 4 bytes by task 24382 on cpu 0:
 signal_group_exit include/linux/sched/signal.h:268 [inline]
 do_group_exit+0x55/0x180 kernel/exit.c:886
 __do_sys_exit_group+0xb/0x10 kernel/exit.c:914
 __se_sys_exit_group+0x5/0x10 kernel/exit.c:912
 __x64_sys_exit_group+0x16/0x20 kernel/exit.c:912
 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: 24382 Comm: syz-executor.3 Not tainted 5.9.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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