proc/sysctl: Don't grab i_lock under sysctl_lock.

Konstantin Khlebnikov <khlebnikov@yandex-team.ru> writes:
> This patch has locking problem. I've got lockdep splat under LTP.
>
> [ 6633.115456] ======================================================
> [ 6633.115502] [ INFO: possible circular locking dependency detected ]
> [ 6633.115553] 4.9.10-debug+ #9 Tainted: G             L
> [ 6633.115584] -------------------------------------------------------
> [ 6633.115627] ksm02/284980 is trying to acquire lock:
> [ 6633.115659]  (&sb->s_type->i_lock_key#4){+.+...}, at: [<ffffffff816bc1ce>] igrab+0x1e/0x80
> [ 6633.115834] but task is already holding lock:
> [ 6633.115882]  (sysctl_lock){+.+...}, at: [<ffffffff817e379b>] unregister_sysctl_table+0x6b/0x110
> [ 6633.116026] which lock already depends on the new lock.
> [ 6633.116026]
> [ 6633.116080]
> [ 6633.116080] the existing dependency chain (in reverse order) is:
> [ 6633.116117]
> -> #2 (sysctl_lock){+.+...}:
> -> #1 (&(&dentry->d_lockref.lock)->rlock){+.+...}:
> -> #0 (&sb->s_type->i_lock_key#4){+.+...}:
>
> d_lock nests inside i_lock
> sysctl_lock nests inside d_lock in d_compare
>
> This patch adds i_lock nesting inside sysctl_lock.

Al Viro <viro@ZenIV.linux.org.uk> replied:
> Once ->unregistering is set, you can drop sysctl_lock just fine.  So I'd
> try something like this - use rcu_read_lock() in proc_sys_prune_dcache(),
> drop sysctl_lock() before it and regain after.  Make sure that no inodes
> are added to the list ones ->unregistering has been set and use RCU list
> primitives for modifying the inode list, with sysctl_lock still used to
> serialize its modifications.
>
> Freeing struct inode is RCU-delayed (see proc_destroy_inode()), so doing
> igrab() is safe there.  Since we don't drop inode reference until after we'd
> passed beyond it in the list, list_for_each_entry_rcu() should be fine.

I agree with Al Viro's analsysis of the situtation.

Fixes: d6cffbbe9a7e ("proc/sysctl: prune stale dentries during unregistering")
Reported-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Tested-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Suggested-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

fs/proc/proc_sysctl.c

@@ -266,21 +266,19 @@ static void proc_sys_prune_dcache(struct ctl_table_header *head)
 	struct inode *inode, *prev = NULL;
 	struct proc_inode *ei;
 
-	list_for_each_entry(ei, &head->inodes, sysctl_inodes) {
+	rcu_read_lock();
+	list_for_each_entry_rcu(ei, &head->inodes, sysctl_inodes) {
 		inode = igrab(&ei->vfs_inode);
 		if (inode) {
-			spin_unlock(&sysctl_lock);
+			rcu_read_unlock();
 			iput(prev);
 			prev = inode;
 			d_prune_aliases(inode);
-			spin_lock(&sysctl_lock);
+			rcu_read_lock();
 		}
 	}
-	if (prev) {
-		spin_unlock(&sysctl_lock);
-		iput(prev);
-		spin_lock(&sysctl_lock);
-	}
+	rcu_read_unlock();
+	iput(prev);
 }
 
 /* called under sysctl_lock, will reacquire if has to wait */
@@ -296,10 +294,10 @@ static void start_unregistering(struct ctl_table_header *p)
 		p->unregistering = &wait;
 		spin_unlock(&sysctl_lock);
 		wait_for_completion(&wait);
-		spin_lock(&sysctl_lock);
 	} else {
 		/* anything non-NULL; we'll never dereference it */
 		p->unregistering = ERR_PTR(-EINVAL);
+		spin_unlock(&sysctl_lock);
 	}
 	/*
 	 * Prune dentries for unregistered sysctls: namespaced sysctls
@@ -310,6 +308,7 @@ static void start_unregistering(struct ctl_table_header *p)
 	 * do not remove from the list until nobody holds it; walking the
 	 * list in do_sysctl() relies on that.
 	 */
+	spin_lock(&sysctl_lock);
 	erase_header(p);
 }
 
@@ -455,11 +454,17 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
 	inode->i_ino = get_next_ino();
 
 	ei = PROC_I(inode);
-	ei->sysctl = head;
-	ei->sysctl_entry = table;
 
 	spin_lock(&sysctl_lock);
-	list_add(&ei->sysctl_inodes, &head->inodes);
+	if (unlikely(head->unregistering)) {
+		spin_unlock(&sysctl_lock);
+		iput(inode);
+		inode = NULL;
+		goto out;
+	}
+	ei->sysctl = head;
+	ei->sysctl_entry = table;
+	list_add_rcu(&ei->sysctl_inodes, &head->inodes);
 	head->count++;
 	spin_unlock(&sysctl_lock);
 
@@ -487,7 +492,7 @@ out:
 void proc_sys_evict_inode(struct inode *inode, struct ctl_table_header *head)
 {
 	spin_lock(&sysctl_lock);
-	list_del(&PROC_I(inode)->sysctl_inodes);
+	list_del_rcu(&PROC_I(inode)->sysctl_inodes);
 	if (!--head->count)
 		kfree_rcu(head, rcu);
 	spin_unlock(&sysctl_lock);