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BUG: KCSAN: data-race in ip6_dst_gc / ip6_dst_gc

write to 0xffff8880af07290c of 4 bytes by task 10308 on cpu 1:
 ip6_dst_gc+0x190/0x200 net/ipv6/route.c:3187
 dst_alloc+0xf4/0x133 net/core/dst.c:85
 ip6_dst_alloc+0x38/0x80 net/ipv6/route.c:355
 ip6_rt_cache_alloc+0x12b/0x480 net/ipv6/route.c:1344
 ip6_pol_route+0x5f4/0x740 net/ipv6/route.c:2223
 ip6_pol_route_output+0x44/0x60 net/ipv6/route.c:2455
 fib6_rule_lookup+0x96/0x450 net/ipv6/fib6_rules.c:114
 ip6_route_output_flags_noref+0x163/0x220 net/ipv6/route.c:2487
 ip6_route_output_flags+0x4d/0x1a0 net/ipv6/route.c:2500
 ip6_dst_lookup_tail+0x25d/0xc50 net/ipv6/ip6_output.c:1052
 ip6_dst_lookup_flow+0x5c/0x110 net/ipv6/ip6_output.c:1153
 rawv6_sendmsg+0x9fb/0x2320 net/ipv6/raw.c:928
 inet_sendmsg+0x69/0x90 net/ipv4/af_inet.c:807
 sock_sendmsg_nosec net/socket.c:652 [inline]
 sock_sendmsg+0x98/0xc0 net/socket.c:672
 kernel_sendmsg+0x48/0x60 net/socket.c:692
 sock_no_sendpage+0xc5/0x100 net/core/sock.c:2740
 kernel_sendpage+0x64/0xa0 net/socket.c:3789
 sock_sendpage+0x64/0x80 net/socket.c:950
 pipe_to_sendpage+0x101/0x190 fs/splice.c:458
 splice_from_pipe_feed fs/splice.c:512 [inline]
 __splice_from_pipe+0x298/0x4a0 fs/splice.c:636
 splice_from_pipe+0xc2/0x100 fs/splice.c:671
 generic_splice_sendpage+0x41/0x60 fs/splice.c:844
 do_splice_from fs/splice.c:863 [inline]
 direct_splice_actor+0x97/0xb0 fs/splice.c:1037
 splice_direct_to_actor+0x22f/0x540 fs/splice.c:992
 do_splice_direct+0x152/0x1d0 fs/splice.c:1080
 do_sendfile+0x396/0x810 fs/read_write.c:1520
 __do_sys_sendfile64 fs/read_write.c:1581 [inline]
 __se_sys_sendfile64 fs/read_write.c:1567 [inline]
 __x64_sys_sendfile64+0x121/0x140 fs/read_write.c:1567
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880af07290c of 4 bytes by task 10274 on cpu 0:
 dst_entries_get_fast include/net/dst_ops.h:48 [inline]
 ip6_dst_gc+0xf4/0x200 net/ipv6/route.c:3176
 dst_alloc+0xf4/0x133 net/core/dst.c:85
 ip6_dst_alloc+0x38/0x80 net/ipv6/route.c:355
 ip6_rt_cache_alloc+0x12b/0x480 net/ipv6/route.c:1344
 ip6_pol_route+0x5f4/0x740 net/ipv6/route.c:2223
 ip6_pol_route_output+0x44/0x60 net/ipv6/route.c:2455
 fib6_rule_lookup+0x96/0x450 net/ipv6/fib6_rules.c:114
 ip6_route_output_flags_noref+0x163/0x220 net/ipv6/route.c:2487
 ip6_route_output_flags+0x4d/0x1a0 net/ipv6/route.c:2500
 ip6_dst_lookup_tail+0x25d/0xc50 net/ipv6/ip6_output.c:1052
 ip6_dst_lookup_flow+0x5c/0x110 net/ipv6/ip6_output.c:1153
 rawv6_sendmsg+0x9fb/0x2320 net/ipv6/raw.c:928
 inet_sendmsg+0x69/0x90 net/ipv4/af_inet.c:807
 sock_sendmsg_nosec net/socket.c:652 [inline]
 sock_sendmsg+0x98/0xc0 net/socket.c:672
 kernel_sendmsg+0x48/0x60 net/socket.c:692
 sock_no_sendpage+0xc5/0x100 net/core/sock.c:2740
 kernel_sendpage+0x64/0xa0 net/socket.c:3789
 sock_sendpage+0x64/0x80 net/socket.c:950
 pipe_to_sendpage+0x101/0x190 fs/splice.c:458
 splice_from_pipe_feed fs/splice.c:512 [inline]
 __splice_from_pipe+0x298/0x4a0 fs/splice.c:636
 splice_from_pipe+0xc2/0x100 fs/splice.c:671
 generic_splice_sendpage+0x41/0x60 fs/splice.c:844
 do_splice_from fs/splice.c:863 [inline]
 direct_splice_actor+0x97/0xb0 fs/splice.c:1037
 splice_direct_to_actor+0x22f/0x540 fs/splice.c:992
 do_splice_direct+0x152/0x1d0 fs/splice.c:1080
 do_sendfile+0x396/0x810 fs/read_write.c:1520
 __do_sys_sendfile64 fs/read_write.c:1581 [inline]
 __se_sys_sendfile64 fs/read_write.c:1567 [inline]
 __x64_sys_sendfile64+0x121/0x140 fs/read_write.c:1567
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 10274 Comm: syz-executor.1 Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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