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BUG: KCSAN: data-race in skb_queue_tail / unix_dgram_poll

write to 0xffff888090f238e0 of 4 bytes by task 27929 on cpu 1:
 __skb_insert include/linux/skbuff.h:1896 [inline]
 __skb_queue_before include/linux/skbuff.h:2001 [inline]
 __skb_queue_tail include/linux/skbuff.h:2034 [inline]
 skb_queue_tail+0x80/0xa0 net/core/skbuff.c:3144
 unix_dgram_sendmsg+0x1003/0x1500 net/unix/af_unix.c:1806
 sock_sendmsg_nosec net/socket.c:652 [inline]
 sock_sendmsg net/socket.c:672 [inline]
 ____sys_sendmsg+0x360/0x4d0 net/socket.c:2352
 ___sys_sendmsg net/socket.c:2406 [inline]
 __sys_sendmmsg+0x322/0x4b0 net/socket.c:2496
 __do_sys_sendmmsg net/socket.c:2525 [inline]
 __se_sys_sendmmsg net/socket.c:2522 [inline]
 __x64_sys_sendmmsg+0x53/0x60 net/socket.c:2522
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888090f238e0 of 4 bytes by task 27935 on cpu 0:
 skb_queue_len include/linux/skbuff.h:1827 [inline]
 unix_recvq_full net/unix/af_unix.c:194 [inline]
 unix_dgram_poll+0x30a/0x3b0 net/unix/af_unix.c:2733
 sock_poll+0x185/0x1a0 net/socket.c:1266
 vfs_poll include/linux/poll.h:90 [inline]
 do_pollfd fs/select.c:869 [inline]
 do_poll fs/select.c:917 [inline]
 do_sys_poll+0x606/0xbc0 fs/select.c:1011
 __do_sys_ppoll fs/select.c:1111 [inline]
 __se_sys_ppoll+0x169/0x1b0 fs/select.c:1091
 __x64_sys_ppoll+0x63/0x70 fs/select.c:1091
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 27935 Comm: syz-executor.2 Not tainted 5.8.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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