doc: Give XDP as example of non-obvious RCU reader/updater pairing
This commit gives an example of non-obvious RCU reader/updater pairing
in the guise of the XDP feature in networking, which calls BPF programs
from network-driver NAPI (softirq) context.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210624160609.292325-4-toke@redhat.com
diff --git a/Documentation/RCU/checklist.rst b/Documentation/RCU/checklist.rst
index 07f6cb8..01cc21f 100644
--- a/Documentation/RCU/checklist.rst
+++ b/Documentation/RCU/checklist.rst
@@ -236,8 +236,15 @@
Mixing things up will result in confusion and broken kernels, and
has even resulted in an exploitable security issue. Therefore,
- when using non-obvious pairs of primitives, commenting is of
- course a must.
+ when using non-obvious pairs of primitives, commenting is
+ of course a must. One example of non-obvious pairing is
+ the XDP feature in networking, which calls BPF programs from
+ network-driver NAPI (softirq) context. BPF relies heavily on RCU
+ protection for its data structures, but because the BPF program
+ invocation happens entirely within a single local_bh_disable()
+ section in a NAPI poll cycle, this usage is safe. The reason
+ that this usage is safe is that readers can use anything that
+ disables BH when updaters use call_rcu() or synchronize_rcu().
8. Although synchronize_rcu() is slower than is call_rcu(), it
usually results in simpler code. So, unless update performance is