Two things:
1. How about expiring client's unix file descriptors by having wineserver writing a single non-zero byte[*] to the server-communication socket when another client modifies the fd? The code to validate fds before use could then be reduced from a server call to a single nonwaiting poll on NtCurrentTeb()->reply_fd.
2. How about using pipes to implement critical sections? InitCriticalSection would create a pipe and write one byte, EnterCritical section would do a poll on the pipe, followed by a read of one byte, and leave critical section would write one byte back to the pipe.
[*] wineserver would write a zero byte when it wished to restart a client after a server call, and the client would need to check this.
Mike