linux-kernel - Re: [PATCH] net: mac80211: tx.c: be sure of 'sdata->vif.type' must be NL80211_IFTYPE_AP when be in NL80211_IFTYPE

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Date:	Sat, 30 Nov 2013 21:08:54 +0100
From:	Johannes Berg <johannes@...solutions.net>
To:	Chen Gang <gang.chen.5i5j@...il.com>
Cc:	"John W. Linville" <linville@...driver.com>,
	rkuo <rkuo@...eaurora.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	linux-wireless@...r.kernel.org, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: mac80211: tx.c: be sure of 'sdata->vif.type' must
 be NL80211_IFTYPE_AP when be in NL80211_IFTYPE_AP case

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

> >>>  	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.

johannes

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