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

From: Andreea Bernat <[email-masked]>
Date: Wed, 20 Aug 2014 20:32:11 +0300

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?
I would like to fully understand this before I remove them.

Thank you,
Andreea

> Thanks
> Christian