On Sat, 2013-11-30 at 21:08 +0100, Johannes Berg wrote:

On Sat, 2013-11-30 at 22:02 +0800, Chen Gang wrote:

quoted

quoted

quoted

quoted

 	case NL80211_IFTYPE_AP:
-		if (sdata->vif.type == NL80211_IFTYPE_AP)
-			chanctx_conf = rcu_dereference(sdata->vif.chanctx_conf);
+		chanctx_conf = rcu_dereference(sdata->vif.chanctx_conf);
 		if (!chanctx_conf)
 			goto fail_rcu;
+try_next:

I don't think that's better than the (fairly obvious) fall-through, and
has a pretty odd goto. Also, depending on the compiler, it still knows
the previous case label and doesn't warn.

Yeah, fall-through is obvious. But check 'A' again just near by "case A"
seems a little strange, and some of compilers (or some of versions) are
really not quit smart enough to know it is not a warning.

Sorry, the paragraph above may lead misunderstanding, I repeated again:

 - fall-through is obvious (although I did not notice it, originally).

 - Check 'A' again just near by "case A" seems a little strange.

 - Some compilers aren't quit smart enough to know 'chanctx_conf' is OK.

I know. If you have any good ideas of how to make it more obvious to the
compiler, I'm all ears, I just don't like any of the solutions offered
so far (and you aren't the first to do so either) :-)

FWIW, I find the label to be odd because if you're familiar with the
code then AP/AP_VLAN *should* be identical except for two special things
that are now linearly & neatly handled in the code (the first being the
4-addr station, the second the chanctx assignment which always has to be
done regardless of 4-addr). IMHO the == check after case should be
enough to make a human reader take a closer look. I understand that you
didn't and that's OK since you were just trying to squelch compile
warnings, but I don't see that this one warrants much attention.

The label/test could be moved to save a couple of lines
of duplicated code.

Maybe:

 net/mac80211/tx.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/net/mac80211/tx.c b/net/mac80211/tx.c
index 70b5a05..b2160f4 100644
--- a/net/mac80211/tx.c
+++ b/net/mac80211/tx.c

@@ -1777,18 +1777,16 @@ netdev_tx_t ieee80211_subif_start_xmit(struct sk_buff *skb,
 		}
 		ap_sdata = container_of(sdata->bss, struct ieee80211_sub_if_data,
 					u.ap);
-		chanctx_conf = rcu_dereference(ap_sdata->vif.chanctx_conf);
-		if (!chanctx_conf)
-			goto fail_rcu;
-		band = chanctx_conf->def.chan->band;
-		if (sta)
-			break;
 		/* fall through */
 	case NL80211_IFTYPE_AP:
-		if (sdata->vif.type == NL80211_IFTYPE_AP)
-			chanctx_conf = rcu_dereference(sdata->vif.chanctx_conf);
+		chanctx_conf = rcu_dereference(ap_sdata->vif.chanctx_conf);
 		if (!chanctx_conf)
 			goto fail_rcu;
+		if (sdata->vif.type == NL80211_IFTYPE_AP_VLAN) {
+			band = chanctx_conf->def.chan->band;
+			if (sta)
+				break;
+		}
 		fc |= cpu_to_le16(IEEE80211_FCTL_FROMDS);
 		/* DA BSSID SA */
 		memcpy(hdr.addr1, skb->data, ETH_ALEN);