net: devmem: use READ_ONCE/WRITE_ONCE on binding->dev

binding->dev is protected on the write-side in
mp_dmabuf_devmem_uninstall() against concurrent writes, but due to the
concurrent bare reads in net_devmem_get_binding() and
validate_xmit_unreadable_skb() it should be wrapped in a
READ_ONCE/WRITE_ONCE pair to make sure no compiler optimizations play
with the underlying register in unforeseen ways.

Doesn't present a critical bug because the known compiler optimizations
don't result in bad behavior. There is no tearing on u64, and load
omissions/invented loads would only break if additional binding->dev
references were inlined together (they aren't right now).

This just more strictly follows the linux memory model (i.e.,
"Lock-Protected Writes With Lockless Reads" in
tools/memory-model/Documentation/access-marking.txt).

Fixes: bd61848 ("net: devmem: Implement TX path")
Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://patch.msgid.link/20260302-devmem-membar-fix-v2-1-5b33c9cbc28b@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: beshleman

net/core/dev.c

@@ -3987,7 +3987,7 @@ static struct sk_buff *validate_xmit_unreadable_skb(struct sk_buff *skb,
 	if (shinfo->nr_frags > 0) {
 		niov = netmem_to_net_iov(skb_frag_netmem(&shinfo->frags[0]));
 		if (net_is_devmem_iov(niov) &&
-		    net_devmem_iov_binding(niov)->dev != dev)
+		    READ_ONCE(net_devmem_iov_binding(niov)->dev) != dev)
 			goto out_free;
 	}
 

net/core/devmem.c

@@ -396,7 +396,8 @@ struct net_devmem_dmabuf_binding *net_devmem_get_binding(struct sock *sk,
 	 * net_device.
 	 */
 	dst_dev = dst_dev_rcu(dst);
-	if (unlikely(!dst_dev) || unlikely(dst_dev != binding->dev)) {
+	if (unlikely(!dst_dev) ||
+	    unlikely(dst_dev != READ_ONCE(binding->dev))) {
 		err = -ENODEV;
 		goto out_unlock;
 	}
@@ -513,7 +514,8 @@ static void mp_dmabuf_devmem_uninstall(void *mp_priv,
 			xa_erase(&binding->bound_rxqs, xa_idx);
 			if (xa_empty(&binding->bound_rxqs)) {
 				mutex_lock(&binding->lock);
-				binding->dev = NULL;
+				ASSERT_EXCLUSIVE_WRITER(binding->dev);
+				WRITE_ONCE(binding->dev, NULL);
 				mutex_unlock(&binding->lock);
 			}
 			break;