Why do you recheck the loop condition here?

I do it to claim that the only race is due to the lost wakeup. We have in the thread finishing DI:

store generation (atomic, works as seq consistent fence)
load pmap_invl_waiters

Without re-check, the order in the waiting thread is

load generation
increment pmap_invl_waiters (seq cst)

which is reverse order to what is needed to make the increment fence to work against the store generation fence. With the re-check, the order becomes

increment pmap_invl_waiters (seq cst)
load generation

You might argue that the callout would close this race anyway, but callout doing the real work means that the thread lost up to 1 tick of the wall time sitting blocked instead of making progress. So leaving callout use only to wakeup race proper is useful.

Thanks. I think this should be explained in a comment.

should be "page's"

I would say, "... below the current thread's number. Prepare to block so that we do not waste CPU cycles or worse, suffer livelock."

"... impossible to block without ..."

"... incrementing ..."

"... thread's invalidation ..." (Note the spelling correction to "invlidation".)

Verb tense: "... which will unblock us if we lose ..."

"Re-check the current ..."