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BUG: KCSAN: data-race in inet_shutdown / sctp_poll

write to 0xffff88809b14e62a of 1 bytes by task 29336 on cpu 1:
 inet_shutdown+0x146/0x200 net/ipv4/af_inet.c:887
 __sys_shutdown net/socket.c:2204 [inline]
 __do_sys_shutdown net/socket.c:2212 [inline]
 __se_sys_shutdown+0xf2/0x140 net/socket.c:2210
 __x64_sys_shutdown+0x2d/0x40 net/socket.c:2210
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88809b14e62a of 1 bytes by task 29328 on cpu 0:
 sctp_poll+0x19c/0x410 net/sctp/socket.c:8380
 sock_poll+0x185/0x1a0 net/socket.c:1265
 vfs_poll include/linux/poll.h:90 [inline]
 ep_item_poll fs/eventpoll.c:887 [inline]
 ep_send_events_proc+0x246/0x620 fs/eventpoll.c:1744
 ep_scan_ready_list+0x13e/0x330 fs/eventpoll.c:714
 ep_send_events fs/eventpoll.c:1788 [inline]
 ep_poll fs/eventpoll.c:1944 [inline]
 do_epoll_wait+0x51f/0x870 fs/eventpoll.c:2333
 __do_sys_epoll_wait fs/eventpoll.c:2343 [inline]
 __se_sys_epoll_wait fs/eventpoll.c:2340 [inline]
 __x64_sys_epoll_wait+0x4d/0x60 fs/eventpoll.c:2340
 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: 0 PID: 29328 Comm: syz-executor.0 Not tainted 5.9.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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