[PATCH] rculist: Add macro list_prev_rcu

[madhuparnabhowmik04@gmail.com]

From: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>

There are instances in the linux kernel where the prev pointer
of a list is accessed.
Unlike list_next_rcu, a similar macro for accessing the prev
pointer was not present.
Therefore, directly accessing the prev pointer was causing
sparse errors.
One such example is the sparse error in fs/nfs/dir.c

error:
fs/nfs/dir.c:2353:14: error: incompatible types in comparison expression (different address spaces):
fs/nfs/dir.c:2353:14:    struct list_head [noderef] <asn:4> *
fs/nfs/dir.c:2353:14:    struct list_head *

The error is caused due to the following line:

lh = rcu_dereference(nfsi->access_cache_entry_lru.prev);

After adding the macro, this error can be fixed as follows:

lh = rcu_dereference(list_prev_rcu(&nfsi->access_cache_entry_lru));

Therefore, we think there is a need to add this macro to rculist.h.

Suggested-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
---
 include/linux/rculist.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index 4b7ae1bf50b3..49eef8437753 100644
--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -40,6 +40,12 @@ static inline void INIT_LIST_HEAD_RCU(struct list_head *list)
  */
 #define list_next_rcu(list)	(*((struct list_head __rcu **)(&(list)->next)))
 
+/*
+ * return the prev pointer of a list_head in an rcu safe
+ * way, we must not access it directly
+ */
+#define list_prev_rcu(list)	(*((struct list_head __rcu **)(&(list)->prev)))
+
 /*
  * Check during list traversal that we are within an RCU reader
  */
-- 
2.17.1