Re: [PATCH v3 3/4] block: add BPF kfuncs to read blkcg io.stat

> diff --git a/kernel/cgroup/bpf_cgroup.c b/kernel/cgroup/bpf_cgroup.c
> index cd28c838dc7b..e253633e8278 100644
> --- a/kernel/cgroup/bpf_cgroup.c
> +++ b/kernel/cgroup/bpf_cgroup.c
> @@ -8,12 +8,50 @@
>  #include <linux/bpf.h>
>  #include <linux/btf_ids.h>
>  #include <linux/cgroup.h>
> +#include <linux/rcupdate.h>
>
> +#ifdef CONFIG_CGROUP_SCHED
>  #include "../sched/sched.h"
> +#endif
>
> -#ifdef CONFIG_CGROUP_SCHED
>  __bpf_kfunc_start_defs();
>
> +/**
> + * bpf_cgroup_css - Get a reference to one controller's css
> + * @cgrp: cgroup to look in
> + * @ssid: controller ID
> + *
> + * The returned css must be released with bpf_css_release().
> + *
> + * Return: The referenced css, or NULL.
> + */
> +__bpf_kfunc struct cgroup_subsys_state *
> +bpf_cgroup_css(struct cgroup *cgrp, int ssid)
> +{
> +	struct cgroup_subsys_state *css;
> +
> +	if (unlikely(ssid < 0 || ssid >= CGROUP_SUBSYS_COUNT))
> +		return NULL;
> +
> +	rcu_read_lock();
> +	css = rcu_dereference(cgrp->subsys[ssid]);
> +	if (css && !css_tryget(css))
> +		css = NULL;
> +	rcu_read_unlock();
> +
> +	return css;
> +}
> +
> +/**
> + * bpf_css_release - Release a css reference
> + * @css: css to release
> + */
> +__bpf_kfunc void bpf_css_release(struct cgroup_subsys_state *css)
> +{
> +	css_put(css);
> +}
> +
> +#ifdef CONFIG_CGROUP_SCHED

[ ... ]

> @@ -30,29 +68,33 @@ bpf_css_to_task_group(struct cgroup_subsys_state *css)
>
>  	return container_of(css, struct task_group, css);
>  }
> +#endif /* CONFIG_CGROUP_SCHED */
>
>  __bpf_kfunc_end_defs();
>
> -BTF_KFUNCS_START(bpf_cpu_cgroup_kfunc_ids)
> +BTF_KFUNCS_START(bpf_cgroup_kfunc_ids)
> +BTF_ID_FLAGS(func, bpf_cgroup_css, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
> +BTF_ID_FLAGS(func, bpf_css_release, KF_RELEASE)
> +#ifdef CONFIG_CGROUP_SCHED
>  BTF_ID_FLAGS(func, bpf_css_to_task_group,
>  	     KF_RCU | KF_RCU_PROTECTED | KF_RET_NULL)
> -BTF_KFUNCS_END(bpf_cpu_cgroup_kfunc_ids)
> +#endif
> +BTF_KFUNCS_END(bpf_cgroup_kfunc_ids)
>
> -static const struct btf_kfunc_id_set bpf_cpu_cgroup_kfunc_set = {
> +static const struct btf_kfunc_id_set bpf_cgroup_kfunc_set = {
>  	.owner		= THIS_MODULE,
> -	.set		= &bpf_cpu_cgroup_kfunc_ids,
> +	.set		= &bpf_cgroup_kfunc_ids,
>  };
>
> -static int __init bpf_cpu_cgroup_kfunc_init(void)
> +static int __init bpf_cgroup_kfunc_init(void)
>  {
>  	int err;
>
>  	err = register_btf_kfunc_id_set(BPF_PROG_TYPE_UNSPEC,
> -					&bpf_cpu_cgroup_kfunc_set);
> +					&bpf_cgroup_kfunc_set);
>  	if (err)
> -		pr_warn("error while registering cpu cgroup kfuncs: %d\n", err);
> +		pr_warn("error while registering cgroup kfuncs: %d\n", err);
>
>  	return err;
>  }
> -late_initcall(bpf_cpu_cgroup_kfunc_init);
> -#endif /* CONFIG_CGROUP_SCHED */
> +late_initcall(bpf_cgroup_kfunc_init);

This isn't a bug, but the patch adds two controller-agnostic kfuncs
(bpf_cgroup_css and bpf_css_release) in kernel/cgroup/bpf_cgroup.c, along
with a three-way rename (bpf_cpu_cgroup_kfunc_ids to bpf_cgroup_kfunc_ids)
and an ifdef restructure of the whole file.

The immediately preceding commit in this series (382c3bd4 "cgroup: add BPF
kfuncs to read a cpu cgroup's stats") already touched this same file under
a "cgroup:" prefix.

Would it be worth splitting the generic css acquire/release and the rename
into a separate "cgroup:" patch, so this one stays scoped to the block
changes?


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/32419919743