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>

1 files changed, 10 insertions, 4 deletions

diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 7bb5cb64f3b8..bb7c2b086fa4 100644
--- a/include/linux/sysctl.h
+++ b/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;