ceph: fix iput race when queueing inode work

If we queue a work item that calls iput(), make sure we ihold() before
attempting to queue work. Otherwise our queued work might miraculously run
before we notice the queue_work() succeeded and call ihold(), allowing the
inode to be destroyed.

That is, instead of

	if (queue_work(...))
		ihold();

we need to do

	ihold();
	if (!queue_work(...))
		iput();
Reported-by: Amon Ott <a.ott@m-privacy.de>
Signed-off-by: Sage Weil <sage@newdream.net>

fs/ceph/inode.c

@@ -1328,12 +1328,13 @@ int ceph_inode_set_size(struct inode *inode, loff_t size)
  */
 void ceph_queue_writeback(struct inode *inode)
 {
+	ihold(inode);
 	if (queue_work(ceph_inode_to_client(inode)->wb_wq,
 		       &ceph_inode(inode)->i_wb_work)) {
 		dout("ceph_queue_writeback %p\n", inode);
-		ihold(inode);
 	} else {
 		dout("ceph_queue_writeback %p failed\n", inode);
+		iput(inode);
 	}
 }
 
@@ -1353,12 +1354,13 @@ static void ceph_writeback_work(struct work_struct *work)
  */
 void ceph_queue_invalidate(struct inode *inode)
 {
+	ihold(inode);
 	if (queue_work(ceph_inode_to_client(inode)->pg_inv_wq,
 		       &ceph_inode(inode)->i_pg_inv_work)) {
 		dout("ceph_queue_invalidate %p\n", inode);
-		ihold(inode);
 	} else {
 		dout("ceph_queue_invalidate %p failed\n", inode);
+		iput(inode);
 	}
 }
 
@@ -1434,13 +1436,14 @@ void ceph_queue_vmtruncate(struct inode *inode)
 {
 	struct ceph_inode_info *ci = ceph_inode(inode);
 
+	ihold(inode);
 	if (queue_work(ceph_sb_to_client(inode->i_sb)->trunc_wq,
 		       &ci->i_vmtruncate_work)) {
 		dout("ceph_queue_vmtruncate %p\n", inode);
-		ihold(inode);
 	} else {
 		dout("ceph_queue_vmtruncate %p failed, pending=%d\n",
 		     inode, ci->i_truncate_pending);
+		iput(inode);
 	}
 }