On Tue, Jun 22, 2021 at 03:55:25PM +0200, Toke Høiland-Jørgensen wrote:

Toke Høiland-Jørgensen [off-list ref] writes:

quoted

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?

I think that my original paragraph needed to have been updated back
when v4.20 came out.  And again when RCU Tasks Trace came out.  ;-)

So I did that updating, then approximated your patch on top of it,
as shown below.  Does this work for you?

							Thanx, Paul

------------------------------------------------------------------------

commit c6ef58907d22f4f327f1e9a637b50a5899aac450
Author: Toke Høiland-Jørgensen [off-list ref]
Date:   Tue Jun 22 11:54:34 2021 -0700

    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: Toke Høiland-Jørgensen [off-list ref]
    Signed-off-by: Paul E. McKenney [off-list ref]

diff --git a/Documentation/RCU/checklist.rst b/Documentation/RCU/checklist.rst
index 4df78f8bd700..f4545b7c9a63 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