[PATCH] sysctl: factor out sysctl_head_next from do_sysctl

The current logic to walk through the list of sysctl table headers is slightly
painful and implement in a way it cannot be used by code outside sysctl.c

I am in the process of implementing a version of the sysctl proc support that
instead of using the proc generic non-caching monster, just uses the existing
sysctl data structure as backing store for building the dcache entries and for
doing directory reads.  To use the existing data structures however I need a
way to get at them.

[akpm@osdl.org: warning fix]
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/sysctl.h

@@ -924,6 +924,10 @@ enum
 #ifdef __KERNEL__
 #include <linux/list.h>
 
+/* For the /proc/sys support */
+extern struct ctl_table_header *sysctl_head_next(struct ctl_table_header *prev);
+extern void sysctl_head_finish(struct ctl_table_header *prev);
+
 extern void sysctl_init(void);
 
 typedef struct ctl_table ctl_table;

kernel/sysctl.c

@@ -1070,6 +1070,42 @@ static void start_unregistering(struct ctl_table_header *p)
 	list_del_init(&p->ctl_entry);
 }
 
+void sysctl_head_finish(struct ctl_table_header *head)
+{
+	if (!head)
+		return;
+	spin_lock(&sysctl_lock);
+	unuse_table(head);
+	spin_unlock(&sysctl_lock);
+}
+
+struct ctl_table_header *sysctl_head_next(struct ctl_table_header *prev)
+{
+	struct ctl_table_header *head;
+	struct list_head *tmp;
+	spin_lock(&sysctl_lock);
+	if (prev) {
+		tmp = &prev->ctl_entry;
+		unuse_table(prev);
+		goto next;
+	}
+	tmp = &root_table_header.ctl_entry;
+	for (;;) {
+		head = list_entry(tmp, struct ctl_table_header, ctl_entry);
+
+		if (!use_table(head))
+			goto next;
+		spin_unlock(&sysctl_lock);
+		return head;
+	next:
+		tmp = tmp->next;
+		if (tmp == &root_table_header.ctl_entry)
+			break;
+	}
+	spin_unlock(&sysctl_lock);
+	return NULL;
+}
+
 void __init sysctl_init(void)
 {
 #ifdef CONFIG_PROC_SYSCTL
@@ -1081,7 +1117,7 @@ void __init sysctl_init(void)
 int do_sysctl(int __user *name, int nlen, void __user *oldval, size_t __user *oldlenp,
 	       void __user *newval, size_t newlen)
 {
-	struct list_head *tmp;
+	struct ctl_table_header *head;
 	int error = -ENOTDIR;
 
 	if (nlen <= 0 || nlen >= CTL_MAXNAME)
@@ -1091,26 +1127,16 @@ int do_sysctl(int __user *name, int nlen, void __user *oldval, size_t __user *ol
 		if (!oldlenp || get_user(old_len, oldlenp))
 			return -EFAULT;
 	}
-	spin_lock(&sysctl_lock);
-	tmp = &root_table_header.ctl_entry;
-	do {
-		struct ctl_table_header *head =
-			list_entry(tmp, struct ctl_table_header, ctl_entry);
-
-		if (!use_table(head))
-			continue;
-
-		spin_unlock(&sysctl_lock);
 
+	for (head = sysctl_head_next(NULL); head;
+			head = sysctl_head_next(head)) {
 		error = parse_table(name, nlen, oldval, oldlenp, 
 					newval, newlen, head->ctl_table);
-
-		spin_lock(&sysctl_lock);
-		unuse_table(head);
-		if (error != -ENOTDIR)
+		if (error != -ENOTDIR) {
+			sysctl_head_finish(head);
 			break;
-	} while ((tmp = tmp->next) != &root_table_header.ctl_entry);
-	spin_unlock(&sysctl_lock);
+		}
+	}
 	return error;
 }