[PATCH v2 06/14] uthread: add cooperative multi-tasking interface

[Heinrich Schuchardt]

On 25.02.25 17:34, Jerome Forissier wrote:
> Add an new internal API called uthread (Kconfig symbol: UTHREAD) which

nits:

%s/an/a/

> provides cooperative multi-tasking. The goal is to be able to improve
> the performance of some parts of U-Boot by overlapping lengthy
> operations, and also implement background jobs in the U-Boot shell.
> Each uthread has its own stack allocated on the heap. The default stack
> size is defined by the UTHREAD_STACK_SIZE symbol and is used when
> uthread_create() receives zero for the stack_sz argument.
>
> The implementation is based on context-switching via initjmp()/setjmp()/
> longjmp() and is inspired from barebox threads [1]. A notion of thread
> group helps with dependencies, such as when a thread needs to block
> until a number of other threads have returned.
>
> The name "uthread" comes from "user-space threads" because the
> scheduling happens with no help from a higher privileged mode, contrary
> to more complex models where kernel threads are defined. But the 'u'
> may as well stand for 'U-Boot' since the bootloader may actually be
> running at any privilege level and the notion of user vs. kernel may
> not make much sense in this context.
>
> [1] https://github.com/barebox/barebox/blob/master/common/bthread.c
>
> Signed-off-by: Jerome Forissier <jerome.forissier at linaro.org>
> ---
>   include/uthread.h |  44 ++++++++++++
>   lib/Kconfig       |  21 ++++++
>   lib/Makefile      |   2 +
>   lib/uthread.c     | 178 ++++++++++++++++++++++++++++++++++++++++++++++
>   4 files changed, 245 insertions(+)
>   create mode 100644 include/uthread.h
>   create mode 100644 lib/uthread.c
>
> diff --git a/include/uthread.h b/include/uthread.h
> new file mode 100644
> index 00000000000..f1f86d210d5
> --- /dev/null
> +++ b/include/uthread.h
> @@ -0,0 +1,44 @@
> +/* SPDX-License-Identifier: GPL-2.0+ */
> +/*
> + * Copyright 2025 Linaro Limited
> + */
> +
> +#include <linux/types.h>
> +
> +#ifndef _UTHREAD_H_
> +#define _UTHREAD_H_
> +
> +#ifdef CONFIG_UTHREAD
> +
> +int uthread_create(void (*fn)(void *), void *arg, size_t stack_sz,
> +		   unsigned int grp_id);

Every function needs a Shinx style documentation.

> +bool uthread_schedule(void);
> +unsigned int uthread_grp_new_id(void);
> +bool uthread_grp_done(unsigned int grp_id);
> +
> +#else
> +
> +static inline int uthread_create(void (*fn)(void *), void *arg, size_t stack_sz,
> +				 unsigned int grp_id)
> +{
> +	fn(arg);
> +	return 0;
> +}
> +
> +static inline bool uthread_schedule(void)
> +{
> +	return false;
> +}
> +
> +static inline unsigned int uthread_grp_new_id(void)
> +{
> +	return 0;
> +}
> +
> +static inline bool uthread_grp_done(unsigned int grp_id)
> +{
> +	return true;
> +}
> +
> +#endif /* CONFIG_UTHREAD */
> +#endif /* _UTHREAD_H_ */
> diff --git a/lib/Kconfig b/lib/Kconfig
> index 1a683dea670..b32740ecbcc 100644
> --- a/lib/Kconfig
> +++ b/lib/Kconfig
> @@ -1255,6 +1255,27 @@ config PHANDLE_CHECK_SEQ
>   	  enable this config option to distinguish them using
>   	  phandles in fdtdec_get_alias_seq() function.
>
> +config UTHREAD
> +	bool "Enable thread support"
> +	depends on HAVE_INITJMP
> +	help
> +	  Implement a simple form of cooperative multi-tasking based on
> +	  context-switching via initjmp(), setjmp() and longjmp(). The
> +	  uthread_ interface enables the main thread of execution to create
> +	  one or more secondary threads and schedule them until they all have
> +	  returned. At any point a thread may suspend its execution and
> +	  schedule another thread, which allows for the efficient multiplexing
> +	  of leghthy operations.
> +
> +config UTHREAD_STACK_SIZE
> +	int "Default uthread stack size"
> +	depends on UTHREAD
> +	default 32768
> +	help
> +	  The default stak size for uthreads. Each uthread has its own stack.
> +	  When the stack_sz argument to uthread_create() is zero then this
> +	  value is used.
> +
>   endmenu
>
>   source "lib/fwu_updates/Kconfig"
> diff --git a/lib/Makefile b/lib/Makefile
> index a7bc2f3134a..3610694de7a 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -164,6 +164,8 @@ obj-$(CONFIG_LIB_ELF) += elf.o
>
>   obj-$(CONFIG_$(PHASE_)SEMIHOSTING) += semihosting.o
>
> +obj-$(CONFIG_UTHREAD) += uthread.o
> +
>   #
>   # Build a fast OID lookup registry from include/linux/oid_registry.h
>   #
> diff --git a/lib/uthread.c b/lib/uthread.c
> new file mode 100644
> index 00000000000..430d1c0de32
> --- /dev/null
> +++ b/lib/uthread.c
> @@ -0,0 +1,178 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (C) 2021 Ahmad Fatoum, Pengutronix
> + * Copyright (C) 2025 Linaro Limited
> + *
> + * An implementation of cooperative multi-tasking inspired from barebox threads
> + * https://github.com/barebox/barebox/blob/master/common/bthread.c
> + */
> +
> +#include <compiler.h>
> +#include <asm/setjmp.h>
> +#include <linux/kernel.h>
> +#include <linux/list.h>
> +#include <malloc.h>
> +#include <stdint.h>
> +#include <uthread.h>
> +
> +static struct uthread {
> +	void (*fn)(void *);
> +	void *arg;
> +	jmp_buf ctx;
> +	void *stack;
> +	bool done;
> +	unsigned int grp_id;
> +	struct list_head list;

The structure and all its components need to be documented.

> +} main_thread = {
> +	.list = LIST_HEAD_INIT(main_thread.list),
> +};
> +
> +static struct uthread *current = &main_thread;
> +
> +/**
> + * uthread_trampoline() - Call the current thread's entry point then resume the
> + * main thread.
> + *
> + * This is a helper function which is used as the @func argument to the inijmp()
> + * function, and ultimately invoked via setjmp(). It does not return, but
> + * instead longjmp()'s back to the main thread.
> + */
> +static void __noreturn uthread_trampoline(void)
> +{
> +	struct uthread *curr = current;
> +
> +	curr->fn(curr->arg);
> +	curr->done = true;
> +	current = &main_thread;
> +	longjmp(current->ctx, 1);
> +	/* Not reached */
> +	while (true)
> +		;
> +}
> +
> +/**
> + * uthread_free() - Free memory used by a uthread object.
> + */
> +static void uthread_free(struct uthread *uthread)
> +{
> +	if (!uthread)
> +		return;
> +	free(uthread->stack);
> +	free(uthread);
> +}
> +
> +/**
> + * uthread_create() - Create a uthread object and make it ready for execution
> + *
> + * Threads are automatically deleted when then return from their entry point.
> + *
> + * @fn: the thread's entry point
> + * @arg: argument passed to the thread's entry point
> + * @stack_sz: stack size for the new thread (in bytes). The stack is allocated
> + * on the heap.
> + * @grp_id: an optional thread group ID that the new thread should belong to
> + * (zero for no group)
> + */

This documentation should be move to the include and the include needs
to be added to doc/api/.

> +int uthread_create(void (*fn)(void *), void *arg, size_t stack_sz,
> +		   unsigned int grp_id)

We should model the functions more like pthreads.

Let the caller provide struct uthread(). This will allow us to implement
pthread_cancel() and phthread_join().

Best regards

Heinrich

> +{
> +	struct uthread *uthread;
> +
> +	if (!stack_sz)
> +		stack_sz = CONFIG_UTHREAD_STACK_SIZE;
> +
> +	uthread = calloc(1, sizeof(*uthread));
> +	if (!uthread)
> +		return -1;
> +
> +	uthread->stack = memalign(16, stack_sz);
> +	if (!uthread->stack)
> +		goto err;
> +
> +	uthread->fn = fn;
> +	uthread->arg = arg;
> +	uthread->grp_id = grp_id;
> +
> +	list_add_tail(&uthread->list, &current->list);
> +
> +	initjmp(uthread->ctx, uthread_trampoline, uthread->stack + stack_sz);
> +
> +	return 0;
> +err:
> +	uthread_free(uthread);
> +	return -1;
> +}
> +
> +/**
> + * uthread_resume() - switch execution to a given thread
> + *
> + * @uthread: the thread object that should be resumed
> + */
> +static void uthread_resume(struct uthread *uthread)
> +{
> +	if (!setjmp(current->ctx)) {
> +		current = uthread;
> +		longjmp(uthread->ctx, 1);
> +	}
> +}
> +
> +/**
> + * uthread_schedule() - yield the CPU to the next runnable thread
> + *
> + * This function is called either by the main thread or any secondary thread
> + * (that is, any thread created via uthread_create()) to switch execution to
> + * the next runnable thread.
> + *
> + * Return: true if a thread was scheduled, false if no runnable thread was found
> + */
> +bool uthread_schedule(void)
> +{
> +	struct uthread *next;
> +	struct uthread *tmp;
> +
> +	if (list_empty(&current->list))
> +	    return false;
> +
> +	list_for_each_entry_safe(next, tmp, &current->list, list) {
> +		if (!next->done) {
> +			uthread_resume(next);
> +			return true;
> +		} else {
> +			/* Found a 'done' thread, free its resources */
> +			list_del(&next->list);
> +			uthread_free(next);
> +		}
> +	}
> +	return false;
> +}
> +
> +/**
> + * uthread_grp_new_id() - return a new ID for a thread group
> + *
> + * Return: the new thread group ID
> + */
> +unsigned int uthread_grp_new_id(void)
> +{
> +	static unsigned int id = 0;
> +
> +	return ++id;
> +}
> +
> +/**
> + * uthread_grp_done() - test if all threads in a group are done
> + *
> + * @grp: the ID of the thread group that should be considered
> + * Return: false if the group contains at least one runnable thread (i.e., one
> + * thread which entry point has not returned yet), true otherwise
> + */
> +bool uthread_grp_done(unsigned int grp_id)
> +{
> +	struct uthread *next;
> +
> +	list_for_each_entry(next, &main_thread.list, list) {
> +		if (next->grp_id == grp_id && !next->done)
> +			return false;
> +	}
> +
> +	return true;
> +}