bpf,docs: Remove references to bpf_cgroup_kptr_get()

The bpf_cgroup_kptr_get() kfunc has been removed, and
bpf_cgroup_acquire() / bpf_cgroup_release() now have the same semantics
as bpf_task_acquire() / bpf_task_release(). This patch updates the BPF
documentation to reflect this.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230411041633.179404-3-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/kfuncs.rst

@@ -572,74 +572,6 @@ bpf_task_release() respectively, so we won't provide examples for them.
 
 ----
 
-You may also acquire a reference to a ``struct cgroup`` kptr that's already
-stored in a map using bpf_cgroup_kptr_get():
-
-.. kernel-doc:: kernel/bpf/helpers.c
-   :identifiers: bpf_cgroup_kptr_get
-
-Here's an example of how it can be used:
-
-.. code-block:: c
-
-	/* struct containing the struct task_struct kptr which is actually stored in the map. */
-	struct __cgroups_kfunc_map_value {
-		struct cgroup __kptr * cgroup;
-	};
-
-	/* The map containing struct __cgroups_kfunc_map_value entries. */
-	struct {
-		__uint(type, BPF_MAP_TYPE_HASH);
-		__type(key, int);
-		__type(value, struct __cgroups_kfunc_map_value);
-		__uint(max_entries, 1);
-	} __cgroups_kfunc_map SEC(".maps");
-
-	/* ... */
-
-	/**
-	 * A simple example tracepoint program showing how a
-	 * struct cgroup kptr that is stored in a map can
-	 * be acquired using the bpf_cgroup_kptr_get() kfunc.
-	 */
-	 SEC("tp_btf/cgroup_mkdir")
-	 int BPF_PROG(cgroup_kptr_get_example, struct cgroup *cgrp, const char *path)
-	 {
-		struct cgroup *kptr;
-		struct __cgroups_kfunc_map_value *v;
-		s32 id = cgrp->self.id;
-
-		/* Assume a cgroup kptr was previously stored in the map. */
-		v = bpf_map_lookup_elem(&__cgroups_kfunc_map, &id);
-		if (!v)
-			return -ENOENT;
-
-		/* Acquire a reference to the cgroup kptr that's already stored in the map. */
-		kptr = bpf_cgroup_kptr_get(&v->cgroup);
-		if (!kptr)
-			/* If no cgroup was present in the map, it's because
-			 * we're racing with another CPU that removed it with
-			 * bpf_kptr_xchg() between the bpf_map_lookup_elem()
-			 * above, and our call to bpf_cgroup_kptr_get().
-			 * bpf_cgroup_kptr_get() internally safely handles this
-			 * race, and will return NULL if the task is no longer
-			 * present in the map by the time we invoke the kfunc.
-			 */
-			return -EBUSY;
-
-		/* Free the reference we just took above. Note that the
-		 * original struct cgroup kptr is still in the map. It will
-		 * be freed either at a later time if another context deletes
-		 * it from the map, or automatically by the BPF subsystem if
-		 * it's still present when the map is destroyed.
-		 */
-		bpf_cgroup_release(kptr);
-
-		return 0;
-        }
-
-----
-
 Other kfuncs available for interacting with ``struct cgroup *`` objects are
 bpf_cgroup_ancestor() and bpf_cgroup_from_id(), allowing callers to access
 the ancestor of a cgroup and find a cgroup by its ID, respectively. Both