CGroup API files: add res_counter_read_u64()

Adds a function for returning the value of a resource counter member, in a
form suitable for use in a cgroup read_u64 control file method.

Signed-off-by: Paul Menage <menage@google.com>
Cc: "Li Zefan" <lizf@cn.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Paul Jackson <pj@sgi.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: "YAMAMOTO Takashi" <yamamoto@valinux.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/res_counter.h

@@ -39,8 +39,9 @@ struct res_counter {
 	spinlock_t lock;
 };
 
-/*
+/**
  * Helpers to interact with userspace
+ * res_counter_read_u64() - returns the value of the specified member.
  * res_counter_read/_write - put/get the specified fields from the
  * res_counter struct to/from the user
  *
@@ -51,6 +52,8 @@ struct res_counter {
  * @pos:     and the offset.
  */
 
+u64 res_counter_read_u64(struct res_counter *counter, int member);
+
 ssize_t res_counter_read(struct res_counter *counter, int member,
 		const char __user *buf, size_t nbytes, loff_t *pos,
 		int (*read_strategy)(unsigned long long val, char *s));

kernel/res_counter.c

@@ -93,6 +93,11 @@ ssize_t res_counter_read(struct res_counter *counter, int member,
 			pos, buf, s - buf);
 }
 
+u64 res_counter_read_u64(struct res_counter *counter, int member)
+{
+	return *res_counter_member(counter, member);
+}
+
 ssize_t res_counter_write(struct res_counter *counter, int member,
 		const char __user *userbuf, size_t nbytes, loff_t *pos,
 		int (*write_strategy)(char *st_buf, unsigned long long *val))