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BUG: KCSAN: data-race in complete_signal / do_group_exit

write to 0xffff8881013212f4 of 4 bytes by task 10643 on cpu 1:
 complete_signal+0x4a5/0x600 kernel/signal.c:1045
 __send_signal+0x680/0x790 kernel/signal.c:1181
 send_signal+0x398/0x3e0 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
 pipe_write+0x817/0xd50 fs/pipe.c:470
 call_write_iter include/linux/fs.h:1901 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x6f0/0x7e0 fs/read_write.c:605
 ksys_write+0xce/0x180 fs/read_write.c:658
 __do_sys_write fs/read_write.c:670 [inline]
 __se_sys_write fs/read_write.c:667 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:667
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8881013212f4 of 4 bytes by task 10641 on cpu 0:
 signal_group_exit include/linux/sched/signal.h:269 [inline]
 do_group_exit+0x51/0x180 kernel/exit.c:905
 __do_sys_exit_group+0xb/0x10 kernel/exit.c:933
 __se_sys_exit_group+0x5/0x10 kernel/exit.c:931
 __x64_sys_exit_group+0x16/0x20 kernel/exit.c:931
 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: 10641 Comm: syz-executor.3 Not tainted 5.11.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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