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bpf,docs: Remove bpf_cpumask_kptr_get() from documentation

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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>
This commit is contained in:
David Vernet 2023-03-16 00:40:28 -05:00 committed by Alexei Starovoitov
parent 1b403ce77d
commit fec2c6d14f
1 changed files with 10 additions and 20 deletions
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30
Documentation/bpf/cpumasks.rst
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@ -117,12 +117,7 @@ For example:
As mentioned and illustrated above, these ``struct bpf_cpumask *`` objects can 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 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 a map, the reference can be removed from the map with bpf_kptr_xchg(), or
opportunistically acquired with bpf_cpumask_kptr_get(): opportunistically acquired using RCU:
.. 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:
.. code-block:: c .. 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 * A simple example tracepoint program showing how a
* struct bpf_cpumask * kptr that is stored in a map can * 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") SEC("tp_btf/cgroup_mkdir")
int BPF_PROG(cgrp_ancestor_example, struct cgroup *cgrp, const char *path) 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) if (!v)
return -ENOENT; return -ENOENT;
bpf_rcu_read_lock();
/* Acquire a reference to the bpf_cpumask * kptr that's already stored in the map. */ /* Acquire a reference to the bpf_cpumask * kptr that's already stored in the map. */
kptr = bpf_cpumask_kptr_get(&v->cpumask); kptr = v->cpumask;
if (!kptr) if (!kptr) {
/* If no bpf_cpumask was present in the map, it's because /* If no bpf_cpumask was present in the map, it's because
* we're racing with another CPU that removed it with * we're racing with another CPU that removed it with
* bpf_kptr_xchg() between the bpf_map_lookup_elem() * bpf_kptr_xchg() between the bpf_map_lookup_elem()
* above, and our call to bpf_cpumask_kptr_get(). * above, and our load of the pointer from the map.
* 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.
*/ */
bpf_rcu_read_unlock();
return -EBUSY; return -EBUSY;
}
/* Free the reference we just took above. Note that the bpf_cpumask_setall(kptr);
* original struct bpf_cpumask * kptr is still in the map. It will bpf_rcu_read_unlock();
* 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);
return 0; return 0;
} }