Re: [PATCH bpf-next v3 03/16] xdp: add proper __rcu annotations to redirect map entries
From: Toke Høiland-Jørgensen <hidden>
Date: 2021-06-22 13:55:34
Also in:
bpf
Subsystem:
documentation, read-copy update (rcu), the rest · Maintainers:
Jonathan Corbet, "Paul E. McKenney", Frederic Weisbecker, Neeraj Upadhyay, Joel Fernandes, Josh Triplett, Boqun Feng, Uladzislau Rezki, Linus Torvalds
Toke Høiland-Jørgensen [off-list ref] writes:
quoted
It would also be great if this scenario in general could be placed under the Documentation/RCU/whatisRCU.rst as an example, so we could refer to the official doc on this, too, if Paul is good with this.I'll take a look and see if I can find a way to fit it in there...
OK, I poked around in Documentation/RCU and decided that the most natural place to put this was in checklist.rst which already talks about local_bh_disable(), but a bit differently. Fixing that up to correspond to what we've been discussing in this thread, and adding a mention of XDP as a usage example, results in the patch below. Paul, WDYT? -Toke
diff --git a/Documentation/RCU/checklist.rst b/Documentation/RCU/checklist.rst
index 1030119294d0..e5bc93e8f9f5 100644
--- a/Documentation/RCU/checklist.rst
+++ b/Documentation/RCU/checklist.rst@@ -226,12 +226,16 @@ over a rather long period of time, but improvements are always welcome! broken kernels, and has even resulted in an exploitable security issue. - One exception to this rule: rcu_read_lock() and rcu_read_unlock() - may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh() - in cases where local bottom halves are already known to be - disabled, for example, in irq or softirq context. Commenting - such cases is a must, of course! And the jury is still out on - whether the increased speed is worth it. + One exception to this rule: a pair of local_bh_disable() / + local_bh_enable() calls function like one big RCU read-side critical + section, so separate rcu_read_lock()s can be omitted in cases where + local bottom halves are already known to be disabled, for example, in + irq or softirq context. Commenting such cases is a must, of course! + One notable example of this usage 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. 8. Although synchronize_rcu() is slower than is call_rcu(), it usually results in simpler code. So, unless update performance is