> I wonder though... wouldn't a safe first way of implementing these new syscalls be to make them actually synchronous?
No, because it visibly changes the semantics. Consider for instance IORING_OP_ACCEPT; if you make it synchronous, and nothing connects to your program, it would wait forever, instead of returning immediately and allowing the program to continue. The file-related opcodes are safer (when used with actual files, instead of network sockets), but still would behave differently for instance with a hanging NFS mount.
