On 2023-12-18 09:33:39 [+0100], Paolo Abeni wrote:

quoted

--- a/net/ipv6/seg6_local.c
+++ b/net/ipv6/seg6_local.c

@@ -1420,41 +1422,44 @@ static int input_action_end_bpf(struct sk_buff *skb,
 	}
 	advance_nextseg(srh, &ipv6_hdr(skb)->daddr);
 
-	/* preempt_disable is needed to protect the per-CPU buffer srh_state,
-	 * which is also accessed by the bpf_lwt_seg6_* helpers
+	/* The access to the per-CPU buffer srh_state is protected by running
+	 * always in softirq context (with disabled BH). On PREEMPT_RT the
+	 * required locking is provided by the following local_lock_nested_bh()
+	 * statement. It is also accessed by the bpf_lwt_seg6_* helpers via
+	 * bpf_prog_run_save_cb().
 	 */
-	preempt_disable();
-	srh_state->srh = srh;
-	srh_state->hdrlen = srh->hdrlen << 3;
-	srh_state->valid = true;
+	scoped_guard(local_lock_nested_bh, &seg6_bpf_srh_states.bh_lock) {
+		srh_state = this_cpu_ptr(&seg6_bpf_srh_states);
+		srh_state->srh = srh;
+		srh_state->hdrlen = srh->hdrlen << 3;
+		srh_state->valid = true;

Here the 'scoped_guard' usage adds a lot of noise to the patch, due to
the added indentation. What about using directly
local_lock_nested_bh()/local_unlock_nested_bh() ?

If this is preferred, sure.

Cheers,

Paolo

Sebastian