cred: add get_cred_rcu()

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@suse.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

include/linux/cred.h

@@ -252,6 +252,17 @@ 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

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;