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BUG: KCSAN: data-race in complete_signal / do_group_exit

read to 0xffff88809c9a5bb4 of 4 bytes by task 10842 on cpu 0:
 do_group_exit+0x7b/0x1c0 kernel/exit.c:904
 __do_sys_exit_group kernel/exit.c:932 [inline]
 __se_sys_exit_group kernel/exit.c:930 [inline]
 __x64_sys_exit_group+0x2e/0x30 kernel/exit.c:930
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff88809c9a5bb4 of 4 bytes by task 10848 on cpu 1:
 complete_signal+0x530/0x5e0 kernel/signal.c:1039
 __send_signal+0x5ea/0x870 kernel/signal.c:1175
 send_signal+0x224/0x2b0 kernel/signal.c:1236
 do_send_sig_info+0x5f/0xc0 kernel/signal.c:1285
 send_sig_info kernel/signal.c:1604 [inline]
 send_sig+0x5f/0x80 kernel/signal.c:1614
 link_pipe fs/splice.c:1655 [inline]
 do_tee+0x39b/0x540 fs/splice.c:1749
 __do_sys_tee fs/splice.c:1774 [inline]
 __se_sys_tee fs/splice.c:1756 [inline]
 __x64_sys_tee+0x1dc/0x1f0 fs/splice.c:1756
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 10848 Comm: syz-executor.5 Not tainted 5.4.0-rc7+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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