| Message ID | 20140817104806.GA14533@ada |
|---|---|
| State | Awaiting Upstream, archived |
| Delegated to: | David Miller |
| Headers | show |
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 <bernat.ada@gmail.com> > --- > 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 :-) . Thanks Christian -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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 <bernat.ada@gmail.com> > > --- > > 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 -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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 <bernat.ada@gmail.com> > > > --- > > > 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> -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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 <bernat.ada@gmail.com> > > > > --- > > > > 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] <http://lxr.free-electrons.com/source/net/mac80211/agg-tx.c#L583> > -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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; }
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. The following Coccinelle semantic patch was used: @@ @@ ( if( (<+... - rcu_dereference + rcu_access_pointer (...) ...+>)) {...} | while( (<+... - rcu_dereference + rcu_access_pointer (...) ...+>)) {...} ) Signed-off-by: Andreea-Cristina Bernat <bernat.ada@gmail.com> --- drivers/net/wireless/ath/carl9170/main.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)