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BUG: KCSAN: data-race in ____sys_sendmsg / fasync_insert_entry

write to 0xffff8881306d8840 of 4 bytes by task 20397 on cpu 1:
 fasync_insert_entry+0x13c/0x1b0 fs/fcntl.c:949
 fasync_add_entry fs/fcntl.c:976 [inline]
 fasync_helper+0x59/0xc0 fs/fcntl.c:994
 sock_fasync+0x5c/0xc0 net/socket.c:1283
 ioctl_fioasync fs/ioctl.c:586 [inline]
 do_vfs_ioctl+0xb6c/0x1410 fs/ioctl.c:990
 __do_sys_ioctl fs/ioctl.c:1067 [inline]
 __se_sys_ioctl+0x83/0x140 fs/ioctl.c:1055
 __x64_sys_ioctl+0x3f/0x50 fs/ioctl.c:1055
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff8881306d8840 of 4 bytes by task 20391 on cpu 0:
 ____sys_sendmsg+0x234/0x4d0 net/socket.c:2335
 ___sys_sendmsg net/socket.c:2404 [inline]
 __sys_sendmmsg+0x315/0x4b0 net/socket.c:2490
 __do_sys_sendmmsg net/socket.c:2519 [inline]
 __se_sys_sendmmsg net/socket.c:2516 [inline]
 __x64_sys_sendmmsg+0x53/0x60 net/socket.c:2516
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 20391 Comm: syz-executor.1 Not tainted 5.13.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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