[PATCH 2/4] cred: add get_cred_rcu()

[NeilBrown]

Sometimes we want to opportunistically get a
ref to a cred in an rcu_read_lock protected section.
get_task_cred() does this, and NFS does as similar thing
with its own credential structures.
To prepare for NFS converting to use 'struct cred' more
uniformly, define get_cred_rcu(), and use it in
get_task_cred().

Signed-off-by: NeilBrown <neilb@xxxxxxxx>
---
 include/linux/cred.h |   11 +++++++++++
 kernel/cred.c        |    2 +-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/linux/cred.h b/include/linux/cred.h
index 6dd51e503f23..69ed76f7d49f 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -247,6 +247,17 @@ static inline const struct cred *get_cred(const struct cred *cred)
 	return get_new_cred(nonconst_cred);
 }
 
+static inline const struct cred *get_cred_rcu(const struct cred *cred)
+{
+	struct cred *nonconst_cred = (struct cred *) cred;
+	if (!cred)
+		return NULL;
+	if (!atomic_inc_not_zero(&nonconst_cred->usage))
+		return NULL;
+	validate_creds(cred);
+	return cred;
+}
+
 /**
  * put_cred - Release a reference to a set of credentials
  * @cred: The credentials to release
diff --git a/kernel/cred.c b/kernel/cred.c
index 4ce75c6fb752..f11aa4e0d2b9 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -195,7 +195,7 @@ const struct cred *get_task_cred(struct task_struct *task)
 	do {
 		cred = __task_cred((task));
 		BUG_ON(!cred);
-	} while (!atomic_inc_not_zero(&((struct cred *)cred)->usage));
+	} while (!get_cred_rcu(cred));
 
 	rcu_read_unlock();
 	return cred;