[PATCH 3/8] fs/proc: take rcu_read_lock() in proc_sys_compare()

[NeilBrown <neil@brown.name>]

proc_sys_compare() is the ->d_compare function for /proc/sys.
It uses rcu_dereference() which assumes the RCU read lock is held and
can complain if it isn't.

However there is no guarantee that this lock is held by d_same_name()
(the caller of ->d_compare).  In particularly d_alloc_parallel() calls
d_same_name() after rcu_read_unlock().

So this patch calls rcu_read_lock() before accessing the inode (which
seems to be the focus of RCU protection here), and drops it afterwards.

Signed-off-by: NeilBrown <neil@brown.name>
---
 fs/proc/proc_sysctl.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 9f1088f138f4..e3d9f36b6699 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -916,19 +916,23 @@ static int proc_sys_compare(const struct dentry *dentry,
 {
 	struct ctl_table_header *head;
 	struct inode *inode;
+	int ret;
 
 	/* Although proc doesn't have negative dentries, rcu-walk means
 	 * that inode here can be NULL */
 	/* AV: can it, indeed? */
+	rcu_read_lock();
 	inode = d_inode_rcu(dentry);
-	if (!inode)
-		return 1;
-	if (name->len != len)
-		return 1;
-	if (memcmp(name->name, str, len))
-		return 1;
-	head = rcu_dereference(PROC_I(inode)->sysctl);
-	return !head || !sysctl_is_seen(head);
+	if (!inode ||
+	    name->len != len ||
+	    memcmp(name->name, str, len)) {
+		ret = 1;
+	} else {
+		head = rcu_dereference(PROC_I(inode)->sysctl);
+		ret = !head || !sysctl_is_seen(head);
+	}
+	rcu_read_unlock();
+	return ret;
 }
 
 static const struct dentry_operations proc_sys_dentry_operations = {
-- 
2.49.0