iget_locked et.al.: make sure we don't return bad inodes

If one thread does iget_locked(), proceeds to try and set
the new inode up and fails, inode will be unhashed and dropped.
However, another thread doing ilookup/iget_locked in the middle
of that would end up finding a half-set-up inode, grabbing
a reference, waiting for it to come unlocked and getting the
resulting bad inode.  It's a race (if that ilookup had been
called just after the failure of setup attempt it wouldn't
have found the sucker at all), particularly unpleasant in
cases when failure is transient/caller-dependent/etc.

While it can be dealt with in the callers, there's no reason
not to handle it in fs/inode.c primitives, especially since
the cost is trivial.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/inode.c

@@ -1019,13 +1019,17 @@ struct inode *iget5_locked(struct super_block *sb, unsigned long hashval,
 {
 	struct hlist_head *head = inode_hashtable + hash(sb, hashval);
 	struct inode *inode;
-
+again:
 	spin_lock(&inode_hash_lock);
 	inode = find_inode(sb, head, test, data);
 	spin_unlock(&inode_hash_lock);
 
 	if (inode) {
 		wait_on_inode(inode);
+		if (unlikely(inode_unhashed(inode))) {
+			iput(inode);
+			goto again;
+		}
 		return inode;
 	}
 
@@ -1062,6 +1066,10 @@ struct inode *iget5_locked(struct super_block *sb, unsigned long hashval,
 		destroy_inode(inode);
 		inode = old;
 		wait_on_inode(inode);
+		if (unlikely(inode_unhashed(inode))) {
+			iput(inode);
+			goto again;
+		}
 	}
 	return inode;
 
@@ -1089,12 +1097,16 @@ struct inode *iget_locked(struct super_block *sb, unsigned long ino)
 {
 	struct hlist_head *head = inode_hashtable + hash(sb, ino);
 	struct inode *inode;
-
+again:
 	spin_lock(&inode_hash_lock);
 	inode = find_inode_fast(sb, head, ino);
 	spin_unlock(&inode_hash_lock);
 	if (inode) {
 		wait_on_inode(inode);
+		if (unlikely(inode_unhashed(inode))) {
+			iput(inode);
+			goto again;
+		}
 		return inode;
 	}
 
@@ -1129,6 +1141,10 @@ struct inode *iget_locked(struct super_block *sb, unsigned long ino)
 		destroy_inode(inode);
 		inode = old;
 		wait_on_inode(inode);
+		if (unlikely(inode_unhashed(inode))) {
+			iput(inode);
+			goto again;
+		}
 	}
 	return inode;
 }
@@ -1264,10 +1280,16 @@ EXPORT_SYMBOL(ilookup5_nowait);
 struct inode *ilookup5(struct super_block *sb, unsigned long hashval,
 		int (*test)(struct inode *, void *), void *data)
 {
-	struct inode *inode = ilookup5_nowait(sb, hashval, test, data);
-
-	if (inode)
+	struct inode *inode;
+again:
+	inode = ilookup5_nowait(sb, hashval, test, data);
+	if (inode) {
 		wait_on_inode(inode);
+		if (unlikely(inode_unhashed(inode))) {
+			iput(inode);
+			goto again;
+		}
+	}
 	return inode;
 }
 EXPORT_SYMBOL(ilookup5);
@@ -1284,13 +1306,18 @@ struct inode *ilookup(struct super_block *sb, unsigned long ino)
 {
 	struct hlist_head *head = inode_hashtable + hash(sb, ino);
 	struct inode *inode;
-
+again:
 	spin_lock(&inode_hash_lock);
 	inode = find_inode_fast(sb, head, ino);
 	spin_unlock(&inode_hash_lock);
 
-	if (inode)
+	if (inode) {
 		wait_on_inode(inode);
+		if (unlikely(inode_unhashed(inode))) {
+			iput(inode);
+			goto again;
+		}
+	}
 	return inode;
 }
 EXPORT_SYMBOL(ilookup);