Concurrency Model

AXL Concurrency Model

AXL targets UEFI, which is cooperatively concurrent and single-threaded on the BSP. There are no OS threads, no preemption, no shared-memory locking to design around. Concurrency comes from callbacks (via AxlLoop) and, where available, work offload onto other cores (via AxlTask).

This doc is the single source of truth for which primitive to reach for. It also records why AXL picked the libuv / GLib / asyncio style of callbacks-plus-stop-tokens over GIL-style threading, stackful coroutines, and protothread macros — so future contributors don’t re-litigate the design.

See src/event/README.md for the prose on the event primitives themselves, and src/loop/README.md for event-loop mechanics.

Also in design: AXL-Runtime.md — a proposal for a CRT0-owned runtime (default loop, Linux-style signal handling, axl_yield() for tight loops, axl_atexit for cleanup). Not yet implemented; current Ctrl-C handling still flows through axl_loop_run’s return code.

A note on naming

“Event” appears three times in AXL docs:

The event loop (AxlLoop) – the dispatcher.
An event source – a thing registered with the loop (timer, idle, raw event, …).
AxlEvent – one kind of source: a one-shot latch.

This mirrors UEFI’s own overload. An AxlEvent is a one-shot latch backed by a UEFI event, and the event loop dispatches them.

The four-axis taxonomy

Every AXL concurrency primitive answers exactly one of four questions. Overlap is minimal and deliberate.

Axis	Primitive	Purpose	Loop integration
Dispatch — “when does my code run?”	`AxlLoop` + sources (timer, timeout, key, idle, proto-notify, raw event)	The event reactor	is the loop
	`AxlDefer`	“Run this soon, on next tick” – escape a constrained callback	requires a running `AxlLoop`
Coordination — “how do I wait for X?”	`axl_wait_for_flag/word`, `axl_wait_for`, `axl_wait_for_with_tick`	Interruptible poll of memory (MMIO status, hardware) or a predicate	spins up a throwaway `AxlLoop`
	`AxlEvent`	Producer signals → waiter resumes (zero polling, UEFI-event-driven)	spins up a throwaway `AxlLoop` for `axl_event_wait_timeout`, or register its handle with a caller-owned loop via `axl_loop_add_event(loop, axl_event_handle(e), ...)`
	`axl_wait_ms`	Interruptible sleep	spins up a throwaway `AxlLoop`
Notification — “how do I tell others?”	`AxlCancellable`	Stop token shared across async ops; cancel once, many ops abort	typed wrapper over `AxlEvent`; ops register its handle on their loop
	`AxlPubsub`	Pub/sub bus – decoupled, many subscribers	delivery is deferred via `AxlDefer` on the caller’s loop
	Direct callback	Coupled point-to-point	caller-defined
	`AxlEventHandle` + `axl_event_signal`	Hand a raw UEFI event to `axl_loop_add_event`; fire via `axl_event_signal(e)`	foreign-event interop (TCP completion tokens, protocol-notify)
Work offload — “run where?”	`AxlTask` pool	Real parallelism on APs (other cores); falls back single-core	AP dispatch, polled via `axl_task_pool_poll()`
	`AxlAsync`	Fire-and-forget AP work with a BSP callback	registers an idle source on the caller’s loop

Decision guide

Pick by what you need to do, not by what primitive looks closest:

I need to…	Use
Run code every N ms	`axl_loop_add_timer`
Run code once after a delay	`axl_loop_add_timeout`
React to keyboard input	`axl_loop_add_key_press`
Do background work between events	`axl_loop_add_idle`
React when a UEFI protocol appears	`axl_loop_add_protocol_notify`
Integrate a firmware-owned EFI_EVENT	`axl_loop_add_event`
Integrate an AxlEvent I own	`axl_loop_add_event(loop, axl_event_handle(e), ...)`
Run code safely from a constrained context	`axl_defer`
Let my async callback wake the main thread	`AxlEvent` + `axl_event_wait_timeout`
Let a caller abort any number of async ops	`AxlCancellable`
Poll a hardware status register (CPU idles)	`axl_wait_for_word` / `axl_wait_for_flag`
Interruptible sleep	`axl_wait_ms`
Wait on a complex condition, driving a state machine	`axl_wait_for_with_tick`
Decouple two modules with named events	`AxlPubsub`
Offload CPU-heavy work to another core	`AxlTask` / `AxlAsync`

Why this model, not another

AXL’s shape is event loop + callbacks + stop-tokens. That’s the model Node.js (libuv), Nginx, pre-async Python asyncio, libev, and GLib all chose — the standard answer for cooperative I/O concurrency in a single-threaded runtime. This section records the alternatives considered and why they don’t fit UEFI.

Why not Python’s GIL model

The GIL is a lock that exists only because CPython has real OS threads and needs to serialize interpreter state. UEFI has no OS threads on the BSP, so there is nothing to lock. The GIL is a workaround born of legacy threading; borrowing the name without the problem would add ceremony without value. Where AXL does touch real parallelism (APs via AxlTask), the primitive is an explicit submit / poll queue — no shared mutable state, no lock needed.

Why not stackful coroutines (fibers / green threads)

Each coroutine gets its own stack. A firmware app might juggle 20 async ops; at 16 KB/stack that’s 320 KB, on a system where every KB matters. Debuggers choke on stack swaps. Lifetime management (what owns the coroutine, when is it reaped) would become a whole new story AXL doesn’t have today. The memory and complexity cost doesn’t buy enough.

Why not stackless coroutines / protothreads

Macro tricks (switch/case / computed goto) to fake yield points. Local variables don’t survive yields, so porting existing C code is painful and error-prone. Debugging a protothread is reading a generated state machine by hand. Worth considering if the codebase were greenfield and async flows were deep — ours are rarely more than three callbacks deep.

Why not `async`/`await` via macros

C doesn’t have it natively. Adding a macro-based emulation (a la libasync or asyncify) buys brevity at the cost of legibility and surprises UEFI developers. The counterweight in AXL is the sync wrappers built on top of the callback primitives: axl_tcp_connect, axl_http_get, axl_wait_*, axl_event_wait_timeout. They let the common flat case be written synchronously; callback nesting only appears where truly async composition is needed. That’s the right pressure valve for firmware.

Where this breaks down

Three-level async flows (connect → TLS handshake → HTTP request) become scope soup. The sync wrappers are the near-term answer. If a concrete pain point surfaces later, a thin AxlFuture / promise layer on top of AxlEvent could compose them with .then() / .all(). Don’t build it speculatively.

Where the primitives live

src/loop/        dispatch       axl-loop.c, axl-defer.c, axl-pubsub.c
src/event/       coordination   axl-event.c, axl-cancellable.c, axl-wait.c
src/task/        offload        axl-task-pool.c, axl-async.c, axl-buf-pool.c

This layout is intentional: each directory corresponds to one axis of the taxonomy. Adding a new concurrency primitive? Pick an axis. If it doesn’t fit any of the four, reconsider whether the primitive earns its weight.

Background reading

GLib GMainLoop — the closest cousin. Same shape: loop + sources + GCancellable.
libuv design — the single-threaded event loop behind Node.js.
Python asyncio pre-await era — protocol + transport callbacks. What async/await replaced.
Linux kernel struct completion (docs) — historical name for what AXL calls AxlEvent.
C++ std::latch — closest C++ analogue of AxlEvent’s one-shot latch semantics.