[PATCH bpf-next 5/5] bpf,docs: Remove bpf_cpumask_kptr_get() from documentation

David Vernet
2023-03-16T01:41:50Z

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>
---
Documentation/bpf/cpumasks.rst | 30 ++++++++++--------------------
1 file changed, 10 insertions(+), 20 deletions(-)

diff --git a/Documentation/bpf/cpumasks.rst b/Documentation/bpf/cpumasks.rst
index 75344cd230e5..41efd8874eeb 100644
--- 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;
}
--
2.39.0