| name | 0.8.4-understand-happens_before |
| description | [0.8.4] transitive closure of program_order and synchronizes_with relations; the ordering guarantee that prevents data |
understand-happens_before
CALL NUMBER: deep_synchronizes_with.happens_before : deep_happens_before_relat(7), concurrency_synchronization_primitives_and_memor(2), deep_c11_memory_model(1)
DEFINITION: transitive closure of program_order and synchronizes_with relations; the ordering guarantee that prevents data races
Invoke this skill to understand happens_before 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_cas (d3): Compare-and-swap operation atomically loading a value, comparing it to an expected value, and storing a new value only if they matched, returning whether the swap occurred.
from concurrency_synchronization_primitives_and_memor
- synchronizes_with (d1): A relation between atomic operations in memory models where a release fence on one thread synchronizes with an acquire fence on another thread, establishing happens-before order across threads.
- atomic_compare_exchange (d2): C++11/C11 atomic operation attempting to replace expected value with desired; returns boolean indicating success; on failure, expected is updated with actual value for retry loops.
from deep_c11_memory_model
- seq_cst_fence (d2): The explicit memory fence with sequentially consistent ordering that provides both acquire and release semantics plus additional ordering constraints to enforce the global total order.
from deep_happens_before_relat
- program_order (d1): The sequential ordering of operations within a single thread as written in the source code, before any concurrent interleaving is considered.
- inter_thread_happens_before (d2): The subset of happens_before relations that cross thread boundaries, established by synchronizes_with connections between release and acquire operations.
- happens_before_order (d3): A transitive, irreflexive partial order over operations in a memory model that defines which operations must appear to precede others from any thread's perspective.
- acquire_fence (d3): A memory ordering primitive that ensures all load and store operations appearing after the fence in program order do not begin until the fence completes and all prior writes are visible.
- release_fence (d3): A memory ordering primitive that ensures all load and store operations appearing before the fence in program order complete before any operations appearing after the fence begin.
- synchronizes_with_acquire_side (d4): The acquire-side endpoint of a synchronizes_with relation: an atomic operation or fence that performs an acquire action, making all subsequently observed writes visible to the acquiring thread.
- synchronizes_with_release_side (d4): The release-side endpoint of a synchronizes_with relation: an atomic operation or fence that performs a release action, making all prior memory operations visible to acquiring threads.
from deep_synchronizes_with
- atomic_load (d2): load operation with memory_order semantics; load with acquire is the endpoint of synchronizes_with from release store
- atomic_store (d2): store operation with memory_order semantics; store with release triggers synchronizes_with on matching acquire
- barrier_arrival (d2): The event when a thread reaches a synchronization barrier; each arrival synchronizes-with all other arrival events at the same barrier instance, enforcing a global rendezvous.
- condvar_broadcast (d2): A condition variable broadcast that wakes all waiting threads; each woken thread's lock acquire synchronizes-with the broadcast operation.
- condvar_signal (d2): signal operation on condition variable; may synchronize_with a condvar_wait on the same condition variable and mutex
- condvar_wait (d2): wait operation on condition variable; releases associated mutex (release) and blocks until signal (acquire); part of condvar synchronizes_with
- full_fence (d2): A memory fence providing both acquire and release semantics; no memory operation on the issuing core may be reordered across the fence in either direction.
- lock_acquire (d2): A synchronization operation that atomically claims exclusive access to a protected region; on most architectures it is an acquire operation establishing a synchronizes-with edge with the matching release.
- lock_release (d2): A synchronization operation that atomically relinquishes exclusive access; it is a release operation establishing a synchronizes-with edge with all subsequent acquires of the same lock.
- memory_order_acquire (d2): Memory ordering barrier ensuring all loads and stores after the barrier in program order cannot be reordered before it, synchronizing with release stores.
- memory_order_release (d2): Memory ordering barrier ensuring all loads and stores before the barrier in program order cannot be reordered after it, making prior writes visible to acquiring threads.
- semaphore_acquire (d2): A decrement of a semaphore counter that claims a permit; when it succeeds the acquire synchronizes-with the release that previously published the permit.
- semaphore_release (d2): An increment of a semaphore counter that releases a permit; the release synchronizes-with any subsequent acquire that consumes that permit.
- signal_delivery (d2): The delivery of a signal to a thread creates a synchronizes-with edge from the last operation before the signal mask change to the first operation in the signal handler.
- thread_creation (d2): The operation of spawning a new thread of execution; the creating thread's operations before the spawn synchronizes-with the new thread's first operation in program order.
CONSUMERS (what needs this)
c11_atomic, dependency_chain, reads_from
Projected from the concurrency synchronization primitives and memory models KB (407 concepts / 477 relations) — consistency-typed by MAP; the facet list after the colon IS the cross-lib dependency web.
(leaf — this is an actual skill.)
