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BUG: KCSAN: data-race in skb_queue_tail / unix_dgram_poll

write to 0xffff888107803ce0 of 4 bytes by task 4977 on cpu 0:
 __skb_insert include/linux/skbuff.h:1917 [inline]
 __skb_queue_before include/linux/skbuff.h:2022 [inline]
 __skb_queue_tail include/linux/skbuff.h:2055 [inline]
 skb_queue_tail+0x80/0xa0 net/core/skbuff.c:3162
 unix_dgram_sendmsg+0xfd9/0x1640 net/unix/af_unix.c:1797
 sock_sendmsg_nosec net/socket.c:652 [inline]
 sock_sendmsg net/socket.c:672 [inline]
 ____sys_sendmsg+0x360/0x4d0 net/socket.c:2345
 ___sys_sendmsg net/socket.c:2399 [inline]
 __sys_sendmsg+0x1ed/0x270 net/socket.c:2432
 __do_sys_sendmsg net/socket.c:2441 [inline]
 __se_sys_sendmsg net/socket.c:2439 [inline]
 __x64_sys_sendmsg+0x42/0x50 net/socket.c:2439
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888107803ce0 of 4 bytes by task 4887 on cpu 1:
 skb_queue_len include/linux/skbuff.h:1848 [inline]
 unix_recvq_full net/unix/af_unix.c:194 [inline]
 unix_dgram_poll+0x30a/0x3b0 net/unix/af_unix.c:2724
 sock_poll+0x23e/0x260 net/socket.c:1244
 vfs_poll include/linux/poll.h:90 [inline]
 ep_item_poll fs/eventpoll.c:840 [inline]
 ep_insert fs/eventpoll.c:1507 [inline]
 do_epoll_ctl+0xff3/0x17d0 fs/eventpoll.c:2127
 __do_sys_epoll_ctl fs/eventpoll.c:2178 [inline]
 __se_sys_epoll_ctl fs/eventpoll.c:2169 [inline]
 __x64_sys_epoll_ctl+0xb3/0xe0 fs/eventpoll.c:2169
 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: 1 PID: 4887 Comm: systemd-journal Not tainted 5.11.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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