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BUG: KCSAN: data-race in ____sys_sendmsg / fasync_insert_entry

write to 0xffff888101245340 of 4 bytes by task 6810 on cpu 0:
 fasync_insert_entry+0x137/0x160 fs/fcntl.c:939
 fasync_add_entry fs/fcntl.c:966 [inline]
 fasync_helper+0x59/0xc0 fs/fcntl.c:984
 sock_fasync+0x5c/0xc0 net/socket.c:1303
 setfl fs/fcntl.c:73 [inline]
 do_fcntl+0xb2b/0xbc0 fs/fcntl.c:347
 __do_sys_fcntl fs/fcntl.c:463 [inline]
 __se_sys_fcntl+0xc2/0x180 fs/fcntl.c:448
 __x64_sys_fcntl+0x3f/0x50 fs/fcntl.c:448
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888101245340 of 4 bytes by task 6811 on cpu 1:
 ____sys_sendmsg+0x234/0x4d0 net/socket.c:2337
 ___sys_sendmsg net/socket.c:2406 [inline]
 __sys_sendmmsg+0x322/0x4b0 net/socket.c:2496
 __do_sys_sendmmsg net/socket.c:2525 [inline]
 __se_sys_sendmmsg net/socket.c:2522 [inline]
 __x64_sys_sendmmsg+0x53/0x60 net/socket.c:2522
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 6811 Comm: syz-executor.3 Not tainted 5.8.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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