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BUG: KCSAN: data-race in do_recvmmsg / sock_alloc_send_pskb

read to 0xffff88809c416770 of 4 bytes by task 8270 on cpu 0:
 sock_error include/net/sock.h:2167 [inline]
 sock_alloc_send_pskb+0xc2/0x530 net/core/sock.c:2224
 sock_alloc_send_skb+0x3b/0x50 net/core/sock.c:2261
 __ip6_append_data.isra.0+0x13f2/0x2070 net/ipv6/ip6_output.c:1511
 ip6_make_skb+0x1df/0x300 net/ipv6/ip6_output.c:1874
 udpv6_sendmsg+0x18b8/0x1c20 net/ipv6/udp.c:1465
 inet6_sendmsg+0x6d/0x90 net/ipv6/af_inet6.c:576
 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 0xffff88809c416770 of 4 bytes by task 8264 on cpu 1:
 do_recvmmsg+0x4de/0x5c0 net/socket.c:2657
 __sys_recvmmsg+0x1ef/0x200 net/socket.c:2680
 __do_sys_recvmmsg net/socket.c:2703 [inline]
 __se_sys_recvmmsg net/socket.c:2696 [inline]
 __x64_sys_recvmmsg+0x89/0xb0 net/socket.c:2696
 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: 8264 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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