> and then the "callback" function just updates the page information in
> the futex block directly - as if it was looked up anew.
yes. And it would also have to rehash the futex queue (which is hashed
along (page,offset), because a FUTEX_WAKE has to find the proper queue -
but it's still very cheap.
this also means that FUTEX_WAIT does not have to make the futex page
writable - just making it present and hashing it along the physical page.
Ie. more robust and less intrusive futexes.
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