2014-08-27 13:27:45

by Andreea Bernat

Subject: [PATCH] carl9170: tx: Replace rcu_assign_pointer() with RCU_INIT_POINTER()

According to RCU_INIT_POINTER()'s block comment 3.a, it can be used if
"3. The referenced data structure has already been exposed to readers either
at compile time or via rcu_assign_pointer() -and-
a. You have not made -any- reader-visible changes to this structure since
then".

This case fulfills the conditions above because between the rcu_dereference()
call (cvif = rcu_dereference(ar->beacon_iter);) and the rcu_assign_pointer()
call there is no update of the "cvif" variable.
Therefore, this patch makes the replacement.

The following Coccinelle semantic patch was used:
@@
identifier v;
@@

v = rcu_dereference(...);
... when != rcu_dereference(...);
when != v = ...;
when != (<+...v...+>)++;
when != \(memcpy\|memset\)(...);
(
- rcu_assign_pointer
+ RCU_INIT_POINTER
(..., v);
|
if(...) {
... when != v = ...;
- rcu_assign_pointer
+ RCU_INIT_POINTER
(..., v);
... when any
}
)

Because there are cases where between a “rcu_dereference()” call and a
“rcu_assign_pointer()” call might be updates of the value that interests us,
the Coccinelle semantic patch ignores them and replaces with
"RCU_INIT_POINTER()" only when the update is not happening.

Signed-off-by: Andreea-Cristina Bernat <[email protected]>
---
drivers/net/wireless/ath/carl9170/tx.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/wireless/ath/carl9170/tx.c b/drivers/net/wireless/ath/carl9170/tx.c
index 4cadfd4..ae86a600 100644
--- a/drivers/net/wireless/ath/carl9170/tx.c
+++ b/drivers/net/wireless/ath/carl9170/tx.c
@@ -1557,7 +1557,7 @@ static struct carl9170_vif_info *carl9170_pick_beaconing_vif(struct ar9170 *ar)
}

out:
- rcu_assign_pointer(ar->beacon_iter, cvif);
+ RCU_INIT_POINTER(ar->beacon_iter, cvif);
return cvif;
}

--
1.9.1

2014-08-27 17:38:27

by Christian Lamparter

Subject: Re: [PATCH] carl9170: tx: Replace rcu_assign_pointer() with RCU_INIT_POINTER()

On Wednesday, August 27, 2014 04:27:30 PM Andreea-Cristina Bernat wrote:
> According to RCU_INIT_POINTER()'s block comment 3.a, it can be used if
> "3. The referenced data structure has already been exposed to readers either
> at compile time or via rcu_assign_pointer() -and-
> a. You have not made -any- reader-visible changes to this structure since
> then".
> [...]
>
> Signed-off-by: Andreea-Cristina Bernat <[email protected]>
Acked-by: Christian Lamparter <[email protected]>

[OT: John, will "carl9170: Remove redundant protection check" make it
in the next round too?]

Regards
Christian