Return-path: Received: from mail-la0-f42.google.com ([209.85.215.42]:48495 "EHLO mail-la0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753351AbaHUUKh (ORCPT ); Thu, 21 Aug 2014 16:10:37 -0400 Date: Thu, 21 Aug 2014 23:10:26 +0300 From: Andreea Bernat To: Christian Lamparter Cc: linville@tuxdriver.com, linux-wireless@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, paulmck@linux.vnet.ibm.com Subject: Re: [PATCH] carl9170: Replace rcu_dereference() with rcu_access_pointer() Message-ID: <20140821201026.GA26428@ada> (sfid-20140821_221059_696877_86CB5E5C) References: <20140817104806.GA14533@ada> <3029076.uaUkTd8nru@debian64> <20140820173210.GA18892@ada> <3042528.E5hT1D5bWz@debian64> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <3042528.E5hT1D5bWz@debian64> Sender: linux-wireless-owner@vger.kernel.org List-ID: On Wed, Aug 20, 2014 at 01:20:02PM -0700, Christian Lamparter wrote: > 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 > > > > --- > > > > 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. Your explanation is clear. I will send a version two of the patch with those lines removed. Thank you, Andreea > > Regards > Christian > > [0] >