linux-kernel - [PATCH] carl9170: tx: Replace rcu_assign_pointer() with RCU_INIT

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Date:	Wed, 27 Aug 2014 16:27:30 +0300
From:	Andreea-Cristina Bernat <bernat.ada@...il.com>
To:	chunkeey@...glemail.com, linville@...driver.com,
	linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc:	paulmck@...ux.vnet.ibm.com
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 <bernat.ada@...il.com>
---
 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

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