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BUG: KCSAN: data-race in skb_dequeue / unix_dgram_sendmsg

read to 0xffff8880a8431ce0 of 4 bytes by task 25723 on cpu 1:
 unix_recvq_full net/unix/af_unix.c:194 [inline]
 unix_dgram_sendmsg+0x9ef/0xc80 net/unix/af_unix.c:1715
 sock_sendmsg_nosec net/socket.c:637 [inline]
 sock_sendmsg+0x9f/0xc0 net/socket.c:657
 ___sys_sendmsg+0x2b7/0x5d0 net/socket.c:2311
 __sys_sendmmsg+0x123/0x350 net/socket.c:2413
 __do_sys_sendmmsg net/socket.c:2442 [inline]
 __se_sys_sendmmsg net/socket.c:2439 [inline]
 __x64_sys_sendmmsg+0x64/0x80 net/socket.c:2439
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff8880a8431ce0 of 4 bytes by task 25727 on cpu 0:
 __skb_unlink include/linux/skbuff.h:2004 [inline]
 __skb_dequeue include/linux/skbuff.h:2024 [inline]
 skb_dequeue+0x71/0x100 net/core/skbuff.c:3040
 skb_queue_purge+0x2f/0x50 net/core/skbuff.c:3077
 unix_dgram_disconnected+0x42/0xe0 net/unix/af_unix.c:474
 unix_dgram_connect+0x210/0x490 net/unix/af_unix.c:1164
 __sys_connect+0x1e9/0x250 net/socket.c:1828
 __do_sys_connect net/socket.c:1839 [inline]
 __se_sys_connect net/socket.c:1836 [inline]
 __x64_sys_connect+0x4c/0x60 net/socket.c:1836
 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: 0 PID: 25727 Comm: syz-executor.0 Not tainted 5.4.0-rc3+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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