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doc: Give XDP as example of non-obvious RCU reader/updater pairing

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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
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Toke Høiland-Jørgensen 2021-06-24 18:05:53 +02:00 committed by Daniel Borkmann
parent 9a145c04a2
commit e74c74f9e5
1 changed files with 9 additions and 2 deletions
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11
Documentation/RCU/checklist.rst
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@ -236,8 +236,15 @@ over a rather long period of time, but improvements are always welcome!
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