0-3-3-understand-memory-order-relaxed-vs-acquire

name	0.3.3-understand-memory_order_relaxed_vs_acquire
description	[0.3.3] The ordering contrast where acquire_semantics prevents subsequent operations from being reordered before the a

understand-memory_order_relaxed_vs_acquire

CALL NUMBER: deep_c11_memory_model.memory_order_relaxed_vs_acquire : deep_synchronizes_with(20), deep_indivisibility_prope(9), deep_happens_before_relat(7), concurrency_synchronization_primitives_and_memor(5) DEFINITION: The ordering contrast where acquire_semantics prevents subsequent operations from being reordered before the acquire, while memory_order_relaxed permits all such reorderings with no barrier effect.

Invoke this skill to understand memory_order_relaxed_vs_acquire 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 (d4): 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

• happens_before_relation (d2): A fundamental ordering relation in memory models defining that if operation A happens-before operation B, then A's effects are visible to B; forms the basis for reasoning about program correctness in concurrent settings.
• synchronizes_with (d2): 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.
• acquire_semantics (d2): Memory ordering semantics ensuring all subsequent loads/stores cannot be reordered before the acquire operation; used for lock acquisition, reading a flag after volatile write.
• memory_order_relaxed (d2): C11/C++11 memory ordering that only guarantees atomicity of the operation with no ordering constraints relative to other operations; allows all reorderings.
• atomic_compare_exchange (d3): 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

• relaxed_coherence_per_location_vs_acquire_global_observation (d1): The observation model contrast where relaxed operations on different variables maintain independent per-variable coherence orders with no cross-variable ordering coordination, while acquire semantics participates in the global sequentially consistent order when mixed with seq_cst operations.
• relaxed_load_value_visibility_timing_vs_acquire_visibility_timing (d1): The visibility timing contrast where a relaxed load may observe any value from the modification order at an undefined time with no guarantee about when the value becomes visible to other threads, while an acquire load observes the value at a specific point in the happens-before order with guaranteed visibility semantics.
• relaxed_no_happens_before_edge_vs_acquire_hb_edge (d1): The ordering contrast where relaxed operations never establish a happens-before edge with operations on other threads regardless of visibility timing, while acquire semantics combined with release on another thread creates a transitive happens-before edge connecting the release writer to the acquire reader.
• relaxed_no_ordering_constraints_vs_acquire_subsequent_constrained (d1): The scope contrast where relaxed operations impose zero ordering constraints on any other operations on any memory location, while acquire semantics constrains all subsequent loads and stores in the same thread to remain after the acquire in program order.
• relaxed_no_synchronizes_with_vs_acquire_synchronizes_with (d1): The synchronization contrast where relaxed atomic operations do not participate in the synchronizes-with relation and establish no cross-thread ordering, while acquire operations on a matching release create a synchronizes-with relationship establishing inter-thread happens-before.
• relaxed_program_order_preservation_vs_acquire_fence_effect (d1): The barrier contrast where relaxed operations preserve intra-thread program order only for the storing thread's own perspective without cross-thread visibility guarantees, while acquire semantics acts as a memory fence that prevents both the compiler and processor from moving subsequent operations before the acquire point.
• relaxed_reordering_freedom_vs_acquire_reordering_prevention (d1): The fundamental reordering contrast where relaxed operations may be freely reordered with respect to other operations on any memory location, whereas acquire semantics forbids any subsequent operation from moving before the acquire fence.
• relaxed_store_buffering_vs_acquire_visibility_guarantee (d1): The buffering contrast where relaxed stores may remain indefinitely in store buffers and become visible to other threads in an order different from program order, while acquire semantics guarantees that all prior writes by the releasing thread become visible to the acquiring thread before the acquire completes.

from deep_happens_before_relat

• acquire_fence (d4): 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 (d4): 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 (d5): 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 (d5): 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.
• program_order (d6): 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 (d7): The subset of happens_before relations that cross thread boundaries, established by synchronizes_with connections between release and acquire operations.
• happens_before_order (d8): 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.

from deep_indivisibility_prope

• isi_atomicity_boundary (d5): The demarcation line separating the before-state from the after-state of a relaxed atomic operation; no observable execution window exists across this boundary.
• isi_operation_completeness (d6): The property that a relaxed atomic operation executes to full completion before any observer can witness its effect or lack thereof; partial execution is undetectable.
• isi_no_in_flight_state (d7): The guaranteed absence of any transient value during operation execution that could be sampled by a concurrent thread; the in-flight state is logically invisible.
• isi_post_state_visibility (d7): The state of the target atomic variable after the operation has fully completed, which is the only alternative state an observer may witness.
• isi_pre_state_visibility (d7): The state of the target atomic variable as it existed before the operation commenced, which is the only alternative state an observer may witness.
• isi_load_visibility_rule (d8): For a relaxed atomic load, the read returns exactly one value from the modification order; no torn read of a partial encoding is observable.
• isi_rmw_visibility_rule (d8): For a relaxed read-modify-write operation, observers see either the complete state before the modification or the complete state after modification; the read-modify-write executes atomically without observable intermediate stages.
• isi_store_visibility_rule (d8): For a relaxed atomic store, any reading thread observes either the complete value that existed before the store or the complete value after the store; torn writes are impossible.
• isi_observer_exclusivity (d8): The constraint that any given observer of a relaxed atomic operation sees exclusively one of the two complete states; mixed or hybrid observations are prohibited.

from deep_synchronizes_with

• atomic_load (d3): load operation with memory_order semantics; load with acquire is the endpoint of synchronizes_with from release store
• atomic_store (d3): store operation with memory_order semantics; store with release triggers synchronizes_with on matching acquire
• barrier_arrival (d3): 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 (d3): A condition variable broadcast that wakes all waiting threads; each woken thread's lock acquire synchronizes-with the broadcast operation.
• condvar_signal (d3): signal operation on condition variable; may synchronize_with a condvar_wait on the same condition variable and mutex
• condvar_wait (d3): wait operation on condition variable; releases associated mutex (release) and blocks until signal (acquire); part of condvar synchronizes_with
• full_fence (d3): 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 (d3): 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 (d3): 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 (d3): 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 (d3): 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 (d3): 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 (d3): An increment of a semaphore counter that releases a permit; the release synchronizes-with any subsequent acquire that consumes that permit.
• signal_delivery (d3): 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 (d3): 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)

acquire_semantics, memory_order_relaxed_atomicity_guarantee

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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.)