netdev - [PATCH net-next] xdp: document xdp_do_flush() before napi_complete

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Date:   Tue, 17 Jan 2023 10:43:05 +0100
From:   Magnus Karlsson <magnus.karlsson@...il.com>
To:     magnus.karlsson@...el.com, bjorn@...nel.org, ast@...nel.org,
        daniel@...earbox.net, netdev@...r.kernel.org,
        jonathan.lemon@...il.com, maciej.fijalkowski@...el.com,
        kuba@...nel.org, toke@...hat.com, pabeni@...hat.com,
        davem@...emloft.net, brouer@...hat.com
Cc:     bpf@...r.kernel.org
Subject: [PATCH net-next] xdp: document xdp_do_flush() before napi_complete_done()

From: Magnus Karlsson <magnus.karlsson@...el.com>

Document in the XDP_REDIRECT manual section that drivers must call
xdp_do_flush() before napi_complete_done(). The two reasons behind
this can be found following the links below.

Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>
Link: https://lore.kernel.org/r/20221220185903.1105011-1-sbohrer@cloudflare.com
Link: https://lore.kernel.org/all/20210624160609.292325-1-toke@redhat.com/
---
 net/core/filter.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index ab811293ae5d..7a2b67893afd 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -4128,9 +4128,13 @@ static const struct bpf_func_proto bpf_xdp_adjust_meta_proto = {
  *    bpf_redirect_info to actually enqueue the frame into a map type-specific
  *    bulk queue structure.
  *
- * 3. Before exiting its NAPI poll loop, the driver will call xdp_do_flush(),
- *    which will flush all the different bulk queues, thus completing the
- *    redirect.
+ * 3. Before exiting its NAPI poll loop, the driver will call
+ *    xdp_do_flush(), which will flush all the different bulk queues,
+ *    thus completing the redirect. Note that xdp_do_flush() must be
+ *    called before napi_complete_done() in the driver, as the
+ *    XDP_REDIRECT logic relies on being inside a single NAPI instance
+ *    through to the xdp_do_flush() call for RCU protection of all
+ *    in-kernel data structures.
  */
 /*
  * Pointers to the map entries will be kept around for this whole sequence of

base-commit: 501543b4fff0ff70bde28a829eb8835081ccef2f
-- 
2.34.1