linux-kernel - [PATCH] block: genhd: Replace rcu_assign_pointer() with RCU_INIT

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Message-ID: <20140827131513.GA31290@ada>
Date:	Wed, 27 Aug 2014 16:15:13 +0300
From:	Andreea-Cristina Bernat <bernat.ada@...il.com>
To:	axboe@...nel.dk, linux-kernel@...r.kernel.org
Cc:	paulmck@...ux.vnet.ibm.com
Subject: [PATCH] block: genhd: 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 and the rcu_assign_pointer() call there is no update of the "part"
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>
---
 block/genhd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/genhd.c b/block/genhd.c
index 3aec057..f7e29d8 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -228,7 +228,7 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
 		part = rcu_dereference(ptbl->part[i]);

 		if (part && sector_in_part(part, sector)) {
-			rcu_assign_pointer(ptbl->last_lookup, part);
+			RCU_INIT_POINTER(ptbl->last_lookup, part);
 			return part;
 		}
 	}
-- 
1.9.1

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