Subject: Re: [PATCH] carl9170: Replace rcu_dereference() with rcu_access_pointer()

From: Christian Lamparter <[email-masked]>
Date: Wed, 20 Aug 2014 13:20:02 -0700 (PDT)

On Wednesday, August 20, 2014 08:32:11 PM Andreea Bernat wrote:
> On Mon, Aug 18, 2014 at 09:29:36PM +0200, Christian Lamparter wrote:
> > On Sunday, August 17, 2014 01:48:07 PM Andreea-Cristina Bernat wrote:
> > > The rcu_dereference() call is used directly in a condition.
> > > Since its return value is never dereferenced it is recommended to use
> > > "rcu_access_pointer()" instead of "rcu_dereference()".
> > > Therefore, this patch makes the replacement.
> > > [...]
> > > Signed-off-by: Andreea-Cristina Bernat <[email-masked]>
> > > ---
> > >  drivers/net/wireless/ath/carl9170/main.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/net/wireless/ath/carl9170/main.c b/drivers/net/wireless/ath/carl9170/main.c
> > > index f8ded84..12018ff 100644
> > > --- a/drivers/net/wireless/ath/carl9170/main.c
> > > +++ b/drivers/net/wireless/ath/carl9170/main.c
> > > @@ -1431,7 +1431,7 @@ static int carl9170_op_ampdu_action(struct ieee80211_hw *hw,
> > >  			return -EOPNOTSUPP;
> > >  
> > >  		rcu_read_lock();
> > > -		if (rcu_dereference(sta_info->agg[tid])) {
> > > +		if (rcu_access_pointer(sta_info->agg[tid])) {
> > >  			rcu_read_unlock();
> > >  			return -EBUSY;
> > >  		}
> > 
> > There's more. The check does not do a whole lot. I think *it* [the check] and the
> > rcu_read_[un]lock [and the return -EBUSY] can be removed completely from the
> > IEEE80211_AMPDU_TX_START code-path in carl9170_op_ampdu_action.
> > 
> > It would be awesome, if you could you make a patch which removes this 
> > unneeded cosmic-ray-protection check :-) .
> 
> Could you tell me why you think that those lines have to be removed?
The carl9170_op_ampdu_action callback is used exclusively by the mac80211
framework to notify the driver about setup and tear down of TX and RX 
aggregation sessions. Hence, mac80211 takes great care of performing
sanity checks and properly serializing calls to the driver's ampdu_action
callback.

Specifically mac80211 already prevents the START of an TX aggregation session,
if the aggregation session is already active [0]. Therefore the driver doesn't
need to perform a similar check as well. This is why:
 - the expression (rcu_dereference(sta_info->agg[tid])) never evaluates to true
 -> the -EBUSY exit path is "dead code"

And without the rcu_dereference(...) the rcu_read protection is not needed
either. So it can be removed for this case as well.

> I would like to fully understand this before I remove them.
Let me know if the explanation above answers sufficient :).
If not, I need some *pointers* to what needs further 
explanation.

Regards
Christian

[0] <http://lxr.free-electrons.com/source/net/mac80211/agg-tx.c#L583>