D573 - Address waits follow the umtx contract

Status: assumed Date: 2026-09-07

The sync-on-address wait and wake are modelled on FreeBSD _umtx_op wait and wake: the wait compares a 64-bit word under the queue's lock and sleeps only while it holds the expected value; the wake ends up to the requested number of sleeps and leaves nothing behind when nobody sleeps. A wait with a non-zero third argument is refused with the placeholder.

Why: a wait that returns without waiting is a busy loop, and no return value fixes it. Comparing under the lock is what stops a wake landing between the read and the sleep. A remembered wake would end a later, unrelated sleep, and waking more than asked hands one job to two workers. The third argument may be a timeout in an unestablished unit, so it is refused rather than read as forever.

Rejected:

• Answering success: the guest spins harder and reaches less.
• Reading the word before taking the lock: loses a wake in the gap.
• Treating any timeout argument as infinite: a plausible answer to a question nobody read.