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BUG: KCSAN: data-race in mptcp_check_for_eof / sk_stream_wait_memory

write to 0xffff8880a69bca6a of 1 bytes by task 8965 on cpu 0:
 mptcp_check_for_eof+0xbc/0x160 net/mptcp/protocol.c:800
 mptcp_worker+0x507/0xe80 net/mptcp/protocol.c:2282
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2275
 worker_thread+0x635/0xb90 kernel/workqueue.c:2421
 kthread+0x1fd/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

read to 0xffff8880a69bca6a of 1 bytes by task 24255 on cpu 1:
 sk_stream_wait_memory+0x48d/0x730 net/core/stream.c:145
 mptcp_sendmsg+0x956/0x1050 net/mptcp/protocol.c:1685
 inet6_sendmsg+0x5f/0x80 net/ipv6/af_inet6.c:638
 sock_sendmsg_nosec net/socket.c:652 [inline]
 sock_sendmsg net/socket.c:672 [inline]
 sock_write_iter+0x1a4/0x200 net/socket.c:999
 call_write_iter include/linux/fs.h:1901 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x6d4/0x7c0 fs/read_write.c:605
 ksys_write+0xce/0x180 fs/read_write.c:658
 __do_sys_write fs/read_write.c:670 [inline]
 __se_sys_write fs/read_write.c:667 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:667
 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: 24255 Comm: syz-executor.3 Not tainted 5.11.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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