> No they're not. Not really, unless you consider disk access and interacting with the page cache/inode cache inside the kernel to be blocking.

The important point is that the kernel takes locks during all those operations, and will wait an unbounded amount of time if those locks are contended.

So really and truly, yes, any synchronous syscall can schedule out for an arbitrary amount of time, no matter what you do.

It's sort of semantic. The word "block" isn't a synonym for "sleep", it has a specific meaning in POSIX. In that meaning, you're correct, they never "block". But in the generic way most people use the term "block", they absolutely do.