Commit e74c74f9 authored by Toke Høiland-Jørgensen's avatar Toke Høiland-Jørgensen Committed by Daniel Borkmann
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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: default avatarPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: default avatarToke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210624160609.292325-4-toke@redhat.com
parent 9a145c04

Documentation/RCU/checklist.rst

+9 −2
Original line number Diff line number Diff line
@@ -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