[altfredd]

> If this flag is specified, the preadv2() system call will return instantly if it would... wait for a lock.

Doesn't this sound a bit different from ordinary short reads?

Receiving EAGAIN usually happens under fairly specific conditions (signal interruption), but I'd imagine, that filesystem code has a great deal of locks.

For example, FUSE filesystems can support signal interruptions via EAGAIN, but they are not guaranteed to. You can end up in situation, when FUSE filesystem hangs, and you can not interrupt the thread, which reads from it. I suspect, that RWF_WAIT is a "fix" for similar situations and not the opposite of default behavior.