[RFC v7 11/21] um: nommu: kernel thread support

[Johannes Berg]

On Tue, 2020-10-06 at 18:44 +0900, Hajime Tazaki wrote:
> nommu mode does not support user processes

I find this really confusing. I'm not sure why you ended up calling this
"nommu mode", but there *are* (still) (other) nommu arches, and they
*do* support userspace processes.

Isn't this really just "LKL mode" or something like that?

>  #define TIF_SYSCALL_TRACE	0	/* syscall trace active */
> @@ -63,6 +85,8 @@ static inline struct thread_info *current_thread_info(void)
>  #define TIF_RESTORE_SIGMASK	7
>  #define TIF_NOTIFY_RESUME	8
>  #define TIF_SECCOMP		9	/* secure computing */
> +#define TIF_SCHED_JB		10
> +#define TIF_HOST_THREAD		11

It'd be nice to document what those mean, and even what "JB" means ... I
saw something about "jump buffer" somewhere, but I have no idea why that
should be a thread flag.

> @@ -16,11 +16,65 @@ struct lkl_jmp_buf {
>   * These operations must be provided by a host library or by the application
>   * itself.
>   *
> + * @sem_alloc - allocate a host semaphore an initialize it to count
> + * @sem_free - free a host semaphore
> + * @sem_up - perform an up operation on the semaphore
> + * @sem_down - perform a down operation on the semaphore
> + *
> + * @mutex_alloc - allocate and initialize a host mutex; the recursive parameter
> + * determines if the mutex is recursive or not
> + * @mutex_free - free a host mutex
> + * @mutex_lock - acquire the mutex
> + * @mutex_unlock - release the mutex
> + *
> + * @thread_create - create a new thread and run f(arg) in its context; returns a
> + * thread handle or 0 if the thread could not be created
> + * @thread_detach - on POSIX systems, free up resources held by
> + * pthreads. Noop on Win32.
> + * @thread_exit - terminates the current thread
> + * @thread_join - wait for the given thread to terminate. Returns 0
> + * for success, -1 otherwise
> + *
> + * @gettid - returns the host thread id of the caller, which need not
> + * be the same as the handle returned by thread_create
> + *
> + * @jmp_buf_set - runs the give function and setups a jump back point by saving
> + * the context in the jump buffer; jmp_buf_longjmp can be called from the give
> + * function or any callee in that function to return back to the jump back
> + * point
> + *
> + * NOTE: we can't return from jmp_buf_set before calling jmp_buf_longjmp or
> + * otherwise the saved context (stack) is not going to be valid, so we must pass
> + * the function that will eventually call longjmp here
> + *
> + * @jmp_buf_longjmp - perform a jump back to the saved jump buffer
> + *
>   * @mem_alloc - allocate memory
>   * @mem_free - free memory

again, kernel-doc.

But I'm starting to doubt the value of having this struct at all. Care
you explain? You're doing everything else already with weak functions,
and you can't very well have _two_ hosts compiled anyway, so what's the
point?

IOW, why isn't this just

void lkl_sem_free(struct lkl_sem *sem);
void lkl_sem_up(struct lkl_sem *sem);
...

and then posix-host.c just includes the header file and implements those
functions?

I don't see any reason for this to be allowed to have multiple variants
linked and then picking them at runtime?

> +/*
> + * This structure is used to get access to the "LKL CPU" that allows us to run

Are you trying to implement SMP? This seems ... rather complex?

> + * Linux code. Because we have to deal with various synchronization requirements
> + * between idle thread, system calls, interrupts, "reentrancy", CPU shutdown,
> + * imbalance wake up (i.e. acquire the CPU from one thread and release it from
> + * another), we can't use a simple synchronization mechanism such as (recursive)
> + * mutex or semaphore. Instead, we use a mutex and a bunch of status data plus a
> + * semaphore.
> + */
> +static struct lkl_cpu {
> +	/* lock that protects the CPU status data */
> +	struct lkl_mutex *lock;
> +	/*
> +	 * Since we must free the cpu lock during shutdown we need a
> +	 * synchronization algorithm between lkl_cpu_shutdown() and the CPU
> +	 * access functions since lkl_cpu_get() gets called from thread
> +	 * destructor callback functions which may be scheduled after
> +	 * lkl_cpu_shutdown() has freed the cpu lock.
> +	 *
> +	 * An atomic counter is used to keep track of the number of running
> +	 * CPU access functions and allow the shutdown function to wait for
> +	 * them.
> +	 *
> +	 * The shutdown functions adds MAX_THREADS to this counter which allows
> +	 * the CPU access functions to check if the shutdown process has
> +	 * started.
> +	 *
> +	 * This algorithm assumes that we never have more the MAX_THREADS
> +	 * requesting CPU access.
> +	 */
> +	#define MAX_THREADS 1000000
> +	unsigned int shutdown_gate;
> +	bool irqs_pending;
> +	/* no of threads waiting the CPU */
> +	unsigned int sleepers;
> +	/* no of times the current thread got the CPU */
> +	unsigned int count;
> +	/* current thread that owns the CPU */
> +	lkl_thread_t owner;
> +	/* semaphore for threads waiting the CPU */
> +	struct lkl_sem *sem;
> +	/* semaphore used for shutdown */
> +	struct lkl_sem *shutdown_sem;
> +} cpu;
> +
> +static int __cpu_try_get_lock(int n)
> +{
> +	lkl_thread_t self;
> +
> +	if (__sync_fetch_and_add(&cpu.shutdown_gate, n) >= MAX_THREADS)
> +		return -2;
> +
> +	lkl_ops->mutex_lock(cpu.lock);
> +
> +	if (cpu.shutdown_gate >= MAX_THREADS)
> +		return -1;
> +
> +	self = lkl_ops->thread_self();
> +
> +	if (cpu.owner && !lkl_ops->thread_equal(cpu.owner, self))
> +		return 0;
> +
> +	cpu.owner = self;
> +	cpu.count++;
> +
> +	return 1;
> +}
> +
> +static void __cpu_try_get_unlock(int lock_ret, int n)
> +{
> +	if (lock_ret >= -1)
> +		lkl_ops->mutex_unlock(cpu.lock);
> +	__sync_fetch_and_sub(&cpu.shutdown_gate, n);
> +}
> +
> +void lkl_cpu_change_owner(lkl_thread_t owner)
> +{
> +	lkl_ops->mutex_lock(cpu.lock);
> +	if (cpu.count > 1)
> +		lkl_bug("bad count while changing owner\n");
> +	cpu.owner = owner;
> +	lkl_ops->mutex_unlock(cpu.lock);
> +}
> +
> +int lkl_cpu_get(void)
> +{
> +	int ret;
> +
> +	ret = __cpu_try_get_lock(1);
> +
> +	while (ret == 0) {
> +		cpu.sleepers++;
> +		__cpu_try_get_unlock(ret, 0);
> +		lkl_ops->sem_down(cpu.sem);
> +		ret = __cpu_try_get_lock(0);
> +	}
> +
> +	__cpu_try_get_unlock(ret, 1);
> +
> +	return ret;
> +}
> +
> +void lkl_cpu_put(void)
> +{
> +	lkl_ops->mutex_lock(cpu.lock);
> +
> +	if (!cpu.count || !cpu.owner ||
> +	    !lkl_ops->thread_equal(cpu.owner, lkl_ops->thread_self()))
> +		lkl_bug("%s: unbalanced put\n", __func__);
> +
> +	while (cpu.irqs_pending && !irqs_disabled()) {
> +		cpu.irqs_pending = false;
> +		lkl_ops->mutex_unlock(cpu.lock);
> +		run_irqs();
> +		lkl_ops->mutex_lock(cpu.lock);
> +	}
> +
> +	if (test_ti_thread_flag(current_thread_info(), TIF_HOST_THREAD) &&
> +	    !single_task_running() && cpu.count == 1) {
> +		if (in_interrupt())
> +			lkl_bug("%s: in interrupt\n", __func__);
> +		lkl_ops->mutex_unlock(cpu.lock);
> +		thread_sched_jb();
> +		return;
> +	}
> +
> +	if (--cpu.count > 0) {
> +		lkl_ops->mutex_unlock(cpu.lock);
> +		return;
> +	}
> +
> +	if (cpu.sleepers) {
> +		cpu.sleepers--;
> +		lkl_ops->sem_up(cpu.sem);
> +	}
> +
> +	cpu.owner = 0;
> +
> +	lkl_ops->mutex_unlock(cpu.lock);
> +}
> +
> +int lkl_cpu_try_run_irq(int irq)
> +{
> +	int ret;
> +
> +	ret = __cpu_try_get_lock(1);
> +	if (!ret) {
> +		set_irq_pending(irq);
> +		cpu.irqs_pending = true;
> +	}
> +	__cpu_try_get_unlock(ret, 1);
> +
> +	return ret;
> +}
> +
> +static void lkl_cpu_shutdown(void)
> +{
> +	__sync_fetch_and_add(&cpu.shutdown_gate, MAX_THREADS);
> +}
> +__uml_exitcall(lkl_cpu_shutdown);
> +
> +void lkl_cpu_wait_shutdown(void)
> +{
> +	lkl_ops->sem_down(cpu.shutdown_sem);
> +	lkl_ops->sem_free(cpu.shutdown_sem);
> +}
> +
> +static void lkl_cpu_cleanup(bool shutdown)
> +{
> +	while (__sync_fetch_and_add(&cpu.shutdown_gate, 0) > MAX_THREADS)
> +		;
> +
> +	if (shutdown)
> +		lkl_ops->sem_up(cpu.shutdown_sem);
> +	else if (cpu.shutdown_sem)
> +		lkl_ops->sem_free(cpu.shutdown_sem);
> +	if (cpu.sem)
> +		lkl_ops->sem_free(cpu.sem);
> +	if (cpu.lock)
> +		lkl_ops->mutex_free(cpu.lock);
> +}

Yeah, what? That's an incomprehensible piece of code. At least add
comments, if it _really_ is necessary?

> +#ifdef doesntwork
> +	/* switch to idle_host_task */
> +	wakeup_idle_host_task();
> +#endif

Well ...

> +/**
> + * This is called before the kernel initializes, so no kernel calls (including
> + * printk) can't be made yet.
> + */

not kernel-doc

try to compile with W=1 :)

johannes