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@xxxxxxxxxx> Signed-off-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx> --- Documentation/RCU/checklist.rst | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/Documentation/RCU/checklist.rst b/Documentation/RCU/checklist.rst index 07f6cb8f674d..01cc21f17f7b 100644 --- a/Documentation/RCU/checklist.rst +++ b/Documentation/RCU/checklist.rst @@ -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 -- 2.32.0
