bpf,docs: Remove bpf_cpumask_kptr_get() from documentation

Now that the kfunc no longer exists, we can remove it and instead
describe how RCU can be used to get a struct bpf_cpumask from a map
value. This patch updates the BPF documentation accordingly.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230316054028.88924-6-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/Documentation/bpf/cpumasks.rst b/Documentation/bpf/cpumasks.rst
index 75344cd..41efd88 100644 (file)
--- a/Documentation/bpf/cpumasks.rst
+++ b/Documentation/bpf/cpumasks.rst
@@ -117,12 +117,7 @@ For example:
 As mentioned and illustrated above, these ``struct bpf_cpumask *`` objects can
 also be stored in a map and used as kptrs. If a ``struct bpf_cpumask *`` is in
 a map, the reference can be removed from the map with bpf_kptr_xchg(), or
-opportunistically acquired with bpf_cpumask_kptr_get():
-
-.. kernel-doc:: kernel/bpf/cpumask.c
-  :identifiers: bpf_cpumask_kptr_get
-
-Here is an example of a ``struct bpf_cpumask *`` being retrieved from a map:
+opportunistically acquired using RCU:
 
 .. code-block:: c
 
@@ -144,7 +139,7 @@ Here is an example of a ``struct bpf_cpumask *`` being retrieved from a map:
        /**
         * A simple example tracepoint program showing how a
         * struct bpf_cpumask * kptr that is stored in a map can
-        * be acquired using the bpf_cpumask_kptr_get() kfunc.
+        * be passed to kfuncs using RCU protection.
         */
        SEC("tp_btf/cgroup_mkdir")
        int BPF_PROG(cgrp_ancestor_example, struct cgroup *cgrp, const char *path)
@@ -158,26 +153,21 @@ Here is an example of a ``struct bpf_cpumask *`` being retrieved from a map:
                if (!v)
                        return -ENOENT;
 
+               bpf_rcu_read_lock();
                /* Acquire a reference to the bpf_cpumask * kptr that's already stored in the map. */
-               kptr = bpf_cpumask_kptr_get(&v->cpumask);
-               if (!kptr)
+               kptr = v->cpumask;
+               if (!kptr) {
                        /* If no bpf_cpumask 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_cpumask_kptr_get().
-                        * bpf_cpumask_kptr_get() internally safely handles this
-                        * race, and will return NULL if the cpumask is no longer
-                        * present in the map by the time we invoke the kfunc.
+                        * above, and our load of the pointer from the map.
                         */
+                       bpf_rcu_read_unlock();
                        return -EBUSY;
+               }
 
-               /* Free the reference we just took above. Note that the
-                * original struct bpf_cpumask * 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_cpumask_release(kptr);
+               bpf_cpumask_setall(kptr);
+               bpf_rcu_read_unlock();
 
                return 0;
        }