0-4-5-understand-mcs-lock-release

name	0.4.5-understand-mcs_lock_release
description	[0.4.5] Operation by which a thread releases the lock; sets own qnode.locked to false to unblock spinning successor; i

understand-mcs_lock_release

CALL NUMBER: deep_spinlock.mcs_lock_release DEFINITION: Operation by which a thread releases the lock; sets own qnode.locked to false to unblock spinning successor; if successor exists in qnode.next, responsibility transfers to successor's own spin; cache-friendly O(1) handoff.

Invoke this skill to understand mcs_lock_release down to its primitives. The RELATIVE ROOT below is the least-fixed-point closure of everything it bundles from — the full import cone, grouped by the lib each prim comes from. Projected from a prover-typed KB (MAP/SWI-Prolog consistency gate): every reference below resolves.

THE RELATIVE ROOT (the import cone, by lib)

from ?

• atomic_exchange (d2): Atomic operation that reads the current value of a variable and writes a new value in a single indivisible step, returning the old value.

from deep_spinlock

• mcs_lock_qnode_locked (d1): Boolean field in qnode; spinning thread reads this; set true on enqueue and cleared false by predecessor upon release; default true indicates lock not yet released.
• mcs_lock_qnode_next (d1): Pointer field in qnode referencing successor qnode; null until predecessor links this node; spinning thread polls this to detect when it has been chained into the queue.
• mcs_lock_wait_for_successor (d1): Sub-protocol when release finds no successor yet linked; thread performing release or a helper spins on qnode.next until non-null; ensures no lock holder exits before its successor is queued.

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Projected from the concurrency synchronization primitives and memory models KB (155 concepts / 137 relations) — consistency-typed by MAP; the facet list after the colon IS the cross-lib dependency web.

(leaf — this is an actual skill.)