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BUG: KCSAN: data-race in complete_signal / do_group_exit

write to 0xffff88803f697a34 of 4 bytes by task 8935 on cpu 1:
 complete_signal+0x4a7/0x5f0 kernel/signal.c:1045
 __send_signal+0x666/0x780 kernel/signal.c:1181
 send_signal+0x39f/0x3d0 kernel/signal.c:1242
 do_send_sig_info+0x9b/0xe0 kernel/signal.c:1291
 send_sig_info kernel/signal.c:1606 [inline]
 send_sig+0x4f/0x60 kernel/signal.c:1616
 splice_pipe_to_pipe fs/splice.c:1467 [inline]
 do_splice+0x740/0x1090 fs/splice.c:1034
 __do_splice fs/splice.c:1137 [inline]
 __do_sys_splice fs/splice.c:1343 [inline]
 __se_sys_splice+0x2a6/0x390 fs/splice.c:1325
 __x64_sys_splice+0x74/0x80 fs/splice.c:1325
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88803f697a34 of 4 bytes by task 8918 on cpu 0:
 signal_group_exit include/linux/sched/signal.h:268 [inline]
 do_group_exit+0x50/0x170 kernel/exit.c:889
 __do_sys_exit_group+0xb/0x10 kernel/exit.c:917
 __se_sys_exit_group+0x5/0x10 kernel/exit.c:915
 __x64_sys_exit_group+0x16/0x20 kernel/exit.c:915
 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: 8918 Comm: syz-executor.3 Not tainted 5.10.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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