net: gro_cells: Reduce lock scope in gro_cell_poll

One GRO-cell device's NAPI callback can nest into the GRO-cell of
another device if the underlying device is also using GRO-cell.
This is the case for IPsec over vxlan.
These two GRO-cells are separate devices. From lockdep's point of view
it is the same because each device is sharing the same lock class and so
it reports a possible deadlock assuming one device is nesting into
itself.

Hold the bh_lock only while accessing gro_cell::napi_skbs in
gro_cell_poll(). This reduces the locking scope and avoids acquiring the
same lock class multiple times.

Fixes: 25718fd ("net: gro_cells: Use nested-BH locking for gro_cell")
Reported-by: Gal Pressman <gal@nvidia.com>
Closes: https://lore.kernel.org/all/66664116-edb8-48dc-ad72-d5223696dd19@nvidia.com/
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20251104153435.ty88xDQt@linutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Sebastian Andrzej Siewior

net/core/gro_cells.c

@@ -60,9 +60,10 @@ static int gro_cell_poll(struct napi_struct *napi, int budget)
 	struct sk_buff *skb;
 	int work_done = 0;
 
-	__local_lock_nested_bh(&cell->bh_lock);
 	while (work_done < budget) {
+		__local_lock_nested_bh(&cell->bh_lock);
 		skb = __skb_dequeue(&cell->napi_skbs);
+		__local_unlock_nested_bh(&cell->bh_lock);
 		if (!skb)
 			break;
 		napi_gro_receive(napi, skb);
@@ -71,7 +72,6 @@ static int gro_cell_poll(struct napi_struct *napi, int budget)
 
 	if (work_done < budget)
 		napi_complete_done(napi, work_done);
-	__local_unlock_nested_bh(&cell->bh_lock);
 	return work_done;
 }