From: "Paul E. McKenney" <paulmck@kernel.org>
To: "Toke Høiland-Jørgensen" <toke@redhat.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>,
Martin KaFai Lau <kafai@fb.com>,
Hangbin Liu <liuhangbin@gmail.com>,
Jesper Dangaard Brouer <brouer@redhat.com>,
Magnus Karlsson <magnus.karlsson@gmail.com>,
Jakub Kicinski <kuba@kernel.org>,
bpf@vger.kernel.org, netdev@vger.kernel.org
Subject: Re: [PATCH bpf-next v3 03/16] xdp: add proper __rcu annotations to redirect map entries
Date: Tue, 22 Jun 2021 13:26:04 -0700 [thread overview]
Message-ID: <20210622202604.GH4397@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <87r1guovg2.fsf@toke.dk>
On Tue, Jun 22, 2021 at 03:55:25PM +0200, Toke Høiland-Jørgensen wrote:
> Toke Høiland-Jørgensen <toke@redhat.com> writes:
>
> >> 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 <toke@redhat.com>
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 <toke@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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
next prev parent reply other threads:[~2021-06-22 20:26 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-17 21:27 [PATCH bpf-next v3 00/16] Clean up and document RCU-based object protection for XDP_REDIRECT Toke Høiland-Jørgensen
2021-06-17 21:27 ` [PATCH bpf-next v3 01/16] rcu: Create an unrcu_pointer() to remove __rcu from a pointer Toke Høiland-Jørgensen
2021-06-17 21:27 ` [PATCH bpf-next v3 02/16] bpf: allow RCU-protected lookups to happen from bh context Toke Høiland-Jørgensen
2021-06-17 21:27 ` [PATCH bpf-next v3 03/16] xdp: add proper __rcu annotations to redirect map entries Toke Høiland-Jørgensen
2021-06-18 4:55 ` Martin KaFai Lau
2021-06-18 23:27 ` Daniel Borkmann
2021-06-21 21:39 ` Toke Høiland-Jørgensen
2021-06-21 22:15 ` Daniel Borkmann
2021-06-21 22:35 ` Toke Høiland-Jørgensen
2021-06-22 8:50 ` Daniel Borkmann
2021-06-22 13:55 ` Toke Høiland-Jørgensen
2021-06-22 20:26 ` Paul E. McKenney [this message]
2021-06-22 21:48 ` Toke Høiland-Jørgensen
2021-06-22 23:19 ` Paul E. McKenney
2021-06-23 10:55 ` Toke Høiland-Jørgensen
2021-06-17 21:27 ` [PATCH bpf-next v3 04/16] ena: remove rcu_read_lock() around XDP program invocation Toke Høiland-Jørgensen
2021-06-17 21:27 ` [PATCH bpf-next v3 05/16] bnxt: " Toke Høiland-Jørgensen
2021-06-17 21:27 ` [PATCH bpf-next v3 06/16] thunderx: " Toke Høiland-Jørgensen
2021-06-17 21:27 ` Toke Høiland-Jørgensen
2021-06-17 21:27 ` [PATCH bpf-next v3 07/16] freescale: " Toke Høiland-Jørgensen
2021-06-17 21:27 ` [PATCH bpf-next v3 08/16] net: intel: " Toke Høiland-Jørgensen
2021-06-17 21:27 ` [Intel-wired-lan] " Toke =?unknown-8bit?q?H=C3=B8iland-J=C3=B8rgensen?=
2021-06-17 21:27 ` [PATCH bpf-next v3 09/16] marvell: " Toke Høiland-Jørgensen
2021-06-17 21:27 ` [PATCH bpf-next v3 10/16] mlx4: " Toke Høiland-Jørgensen
2021-06-17 21:27 ` [PATCH bpf-next v3 11/16] nfp: " Toke Høiland-Jørgensen
2021-06-17 21:27 ` [PATCH bpf-next v3 12/16] qede: " Toke Høiland-Jørgensen
2021-06-17 21:27 ` [PATCH bpf-next v3 13/16] sfc: " Toke Høiland-Jørgensen
2021-06-17 21:27 ` [PATCH bpf-next v3 14/16] netsec: " Toke Høiland-Jørgensen
2021-06-17 21:27 ` [PATCH bpf-next v3 15/16] stmmac: " Toke Høiland-Jørgensen
2021-06-18 9:47 ` Wong Vee Khee
2021-06-17 21:27 ` [PATCH bpf-next v3 16/16] net: ti: " Toke Høiland-Jørgensen
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