Re: [PATCH v3 11/19] unwind: Add deferred user space unwinding API

[Peter Zijlstra]

On Mon, Oct 28, 2024 at 02:47:38PM -0700, Josh Poimboeuf wrote:

> +static void unwind_user_task_work(struct callback_head *head)
> +{
...
> +	guard(rwsem_read)(&callbacks_rwsem);
> +
> +	for_each_set_bit(i, &pending, UNWIND_MAX_CALLBACKS) {
> +		if (callbacks[i])
> +			callbacks[i]->func(&trace, cookie, privs[i]);
> +	}

I'm fairly sure people will come with pitchforks for that read-lock
there. They scale like shit on big systems. Please use SRCU or somesuch.

> +}
> +
> +int unwind_user_register(struct unwind_callback *callback, unwind_callback_t func)
> +{
> +	scoped_guard(rwsem_write, &callbacks_rwsem) {
> +		for (int i = 0; i < UNWIND_MAX_CALLBACKS; i++) {
> +			if (!callbacks[i]) {
> +				callback->func = func;
> +				callback->idx = i;
> +				callbacks[i] = callback;
> +				return 0;
> +			}
> +		}
> +	}
> +
> +	callback->func = NULL;
> +	callback->idx = -1;
> +	return -ENOSPC;
> +}
> +
> +int unwind_user_unregister(struct unwind_callback *callback)
> +{
> +	if (callback->idx < 0)
> +		return -EINVAL;
> +
> +	scoped_guard(rwsem_write, &callbacks_rwsem)
> +		callbacks[callback->idx] = NULL;
> +
> +	callback->func = NULL;
> +	callback->idx = -1;
> +
> +	return 0;
> +}