linux-kernel - [PATCH bpf-next v2 5/5] bpf,docs: Remove bpf_cpumask_kptr

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Message-Id: <20230316054028.88924-6-void@manifault.com>
Date:   Thu, 16 Mar 2023 00:40:28 -0500
From:   David Vernet <void@...ifault.com>
To:     bpf@...r.kernel.org
Cc:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        martin.lau@...ux.dev, song@...nel.org, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org, sdf@...gle.com,
        haoluo@...gle.com, jolsa@...nel.org, linux-kernel@...r.kernel.org,
        kernel-team@...a.com
Subject: [PATCH bpf-next v2 5/5] 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@...ifault.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