netdev - [PATCH bpf-next v4 03/19] doc: Give XDP as example of non-obvious RCU reader/updater pairing

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Date:   Wed, 23 Jun 2021 13:07:11 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     bpf@...r.kernel.org, netdev@...r.kernel.org
Cc:     Martin KaFai Lau <kafai@...com>,
        Hangbin Liu <liuhangbin@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Toke Høiland-Jørgensen <toke@...hat.com>
Subject: [PATCH bpf-next v4 03/19] 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@...nel.org>
Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
---
 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