unfuck proc_sysctl ->d_compare()

a) struct inode is not going to be freed under ->d_compare();
however, the thing PROC_I(inode)->sysctl points to just might.
Fortunately, it's enough to make freeing that sucker delayed,
provided that we don't step on its ->unregistering, clear
the pointer to it in PROC_I(inode) before dropping the reference
and check if it's NULL in ->d_compare().

b) I'm not sure that we *can* walk into NULL inode here (we recheck
dentry->seq between verifying that it's still hashed / fetching
dentry->d_inode and passing it to ->d_compare() and there's no
negative hashed dentries in /proc/sys/*), but if we can walk into
that, we really should not have ->d_compare() return 0 on it!
Said that, I really suspect that this check can be simply killed.
Nick?
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/proc/inode.c

@@ -27,6 +27,7 @@
 static void proc_evict_inode(struct inode *inode)
 {
 	struct proc_dir_entry *de;
+	struct ctl_table_header *head;
 
 	truncate_inode_pages(&inode->i_data, 0);
 	end_writeback(inode);
@@ -38,8 +39,11 @@ static void proc_evict_inode(struct inode *inode)
 	de = PROC_I(inode)->pde;
 	if (de)
 		pde_put(de);
-	if (PROC_I(inode)->sysctl)
-		sysctl_head_put(PROC_I(inode)->sysctl);
+	head = PROC_I(inode)->sysctl;
+	if (head) {
+		rcu_assign_pointer(PROC_I(inode)->sysctl, NULL);
+		sysctl_head_put(head);
+	}
 }
 
 struct vfsmount *proc_mnt;

fs/proc/proc_sysctl.c

@@ -408,15 +408,18 @@ static int proc_sys_compare(const struct dentry *parent,
 		const struct dentry *dentry, const struct inode *inode,
 		unsigned int len, const char *str, const struct qstr *name)
 {
+	struct ctl_table_header *head;
 	/* Although proc doesn't have negative dentries, rcu-walk means
 	 * that inode here can be NULL */
+	/* AV: can it, indeed? */
 	if (!inode)
-		return 0;
+		return 1;
 	if (name->len != len)
 		return 1;
 	if (memcmp(name->name, str, len))
 		return 1;
-	return !sysctl_is_seen(PROC_I(inode)->sysctl);
+	head = rcu_dereference(PROC_I(inode)->sysctl);
+	return !head || !sysctl_is_seen(head);
 }
 
 static const struct dentry_operations proc_sys_dentry_operations = {

include/linux/sysctl.h

@@ -25,6 +25,7 @@
 #include <linux/kernel.h>
 #include <linux/types.h>
 #include <linux/compiler.h>
+#include <linux/rcupdate.h>
 
 struct completion;
 
@@ -1037,10 +1038,15 @@ struct ctl_table_root {
    struct ctl_table trees. */
 struct ctl_table_header
 {
-	struct ctl_table *ctl_table;
-	struct list_head ctl_entry;
-	int used;
-	int count;
+	union {
+		struct {
+			struct ctl_table *ctl_table;
+			struct list_head ctl_entry;
+			int used;
+			int count;
+		};
+		struct rcu_head rcu;
+	};
 	struct completion *unregistering;
 	struct ctl_table *ctl_table_arg;
 	struct ctl_table_root *root;

kernel/sysctl.c

@@ -194,9 +194,9 @@ static int sysrq_sysctl_handler(ctl_table *table, int write,
 static struct ctl_table root_table[];
 static struct ctl_table_root sysctl_table_root;
 static struct ctl_table_header root_table_header = {
-	.count = 1,
+	{{.count = 1,
 	.ctl_table = root_table,
-	.ctl_entry = LIST_HEAD_INIT(sysctl_table_root.default_set.list),
+	.ctl_entry = LIST_HEAD_INIT(sysctl_table_root.default_set.list),}},
 	.root = &sysctl_table_root,
 	.set = &sysctl_table_root.default_set,
 };
@@ -1567,11 +1567,16 @@ void sysctl_head_get(struct ctl_table_header *head)
 	spin_unlock(&sysctl_lock);
 }
 
+static void free_head(struct rcu_head *rcu)
+{
+	kfree(container_of(rcu, struct ctl_table_header, rcu));
+}
+
 void sysctl_head_put(struct ctl_table_header *head)
 {
 	spin_lock(&sysctl_lock);
 	if (!--head->count)
-		kfree(head);
+		call_rcu(&head->rcu, free_head);
 	spin_unlock(&sysctl_lock);
 }
 
@@ -1948,10 +1953,10 @@ void unregister_sysctl_table(struct ctl_table_header * header)
 	start_unregistering(header);
 	if (!--header->parent->count) {
 		WARN_ON(1);
-		kfree(header->parent);
+		call_rcu(&header->parent->rcu, free_head);
 	}
 	if (!--header->count)
-		kfree(header);
+		call_rcu(&header->rcu, free_head);
 	spin_unlock(&sysctl_lock);
 }