Martin KaFai Lau [off-list ref] writes:

On Wed, Jun 09, 2021 at 12:33:12PM +0200, Toke Høiland-Jørgensen wrote:

quoted

Some of the XDP helpers (in particular, xdp_do_redirect()) will get a
struct net_device reference using dev_get_by_index_rcu(). These are called
from a NAPI poll context, which means the RCU reference liveness is ensured
by local_bh_disable(). Add rcu_read_lock_bh_held() as a condition to the
RCU list traversal in dev_get_by_index_rcu() so lockdep understands that
the dereferences are safe from *both* an rcu_read_lock() *and* with
local_bh_disable().

Signed-off-by: Toke Høiland-Jørgensen <redacted>
---
 net/core/dev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index febb23708184..a499c5ffe4a5 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -1002,7 +1002,7 @@ struct net_device *dev_get_by_index_rcu(struct net *net, int ifindex)
 	struct net_device *dev;
 	struct hlist_head *head = dev_index_hash(net, ifindex);
 
-	hlist_for_each_entry_rcu(dev, head, index_hlist)
+	hlist_for_each_entry_rcu(dev, head, index_hlist, rcu_read_lock_bh_held())

Is it needed?  hlist_for_each_entry_rcu() checks for
rcu_read_lock_any_held().  Did lockdep complain?

Ah, yes, I think you're right. I totally missed that
rcu_read_lock_any_held() includes a '!preemptible()' check at the end.
I'll drop this patch, then!

-Toke