ext4: fix ext4_flush_completed_IO wait semantics

BUG #1) All places where we call ext4_flush_completed_IO are broken
    because buffered io and DIO/AIO goes through three stages
    1) submitted io,
    2) completed io (in i_completed_io_list) conversion pended
    3) finished  io (conversion done)
    And by calling ext4_flush_completed_IO we will flush only
    requests which were in (2) stage, which is wrong because:
     1) punch_hole and truncate _must_ wait for all outstanding unwritten io
      regardless to it's state.
     2) fsync and nolock_dio_read should also wait because there is
        a time window between end_page_writeback() and ext4_add_complete_io()
        As result integrity fsync is broken in case of buffered write
        to fallocated region:
        fsync                                      blkdev_completion
	 ->filemap_write_and_wait_range
                                                   ->ext4_end_bio
                                                     ->end_page_writeback
          <-- filemap_write_and_wait_range return
	 ->ext4_flush_completed_IO
   	 sees empty i_completed_io_list but pended
   	 conversion still exist
                                                     ->ext4_add_complete_io

BUG #2) Race window becomes wider due to the 'ext4: completed_io
locking cleanup V4' patch series

This patch make following changes:
1) ext4_flush_completed_io() now first try to flush completed io and when
   wait for any outstanding unwritten io via ext4_unwritten_wait()
2) Rename function to more appropriate name.
3) Assert that all callers of ext4_flush_unwritten_io should hold i_mutex to
   prevent endless wait

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

fs/ext4/ext4.h

@@ -1947,7 +1947,7 @@ extern void ext4_htree_free_dir_info(struct dir_private_info *p);
 
 /* fsync.c */
 extern int ext4_sync_file(struct file *, loff_t, loff_t, int);
-extern int ext4_flush_completed_IO(struct inode *);
+extern int ext4_flush_unwritten_io(struct inode *);
 
 /* hash.c */
 extern int ext4fs_dirhash(const char *name, int len, struct
@@ -2371,6 +2371,7 @@ extern const struct file_operations ext4_dir_operations;
 extern const struct inode_operations ext4_file_inode_operations;
 extern const struct file_operations ext4_file_operations;
 extern loff_t ext4_llseek(struct file *file, loff_t offset, int origin);
+extern void ext4_unwritten_wait(struct inode *inode);
 
 /* namei.c */
 extern const struct inode_operations ext4_dir_inode_operations;

fs/ext4/extents.c

@@ -4268,7 +4268,7 @@ void ext4_ext_truncate(struct inode *inode)
 	 * finish any pending end_io work so we won't run the risk of
 	 * converting any truncated blocks to initialized later
 	 */
-	ext4_flush_completed_IO(inode);
+	ext4_flush_unwritten_io(inode);
 
 	/*
 	 * probably first extent we're gonna free will be last in block
@@ -4847,10 +4847,10 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
 
 	/* Wait all existing dio workers, newcomers will block on i_mutex */
 	ext4_inode_block_unlocked_dio(inode);
-	inode_dio_wait(inode);
-	err = ext4_flush_completed_IO(inode);
+	err = ext4_flush_unwritten_io(inode);
 	if (err)
 		goto out_dio;
+	inode_dio_wait(inode);
 
 	credits = ext4_writepage_trans_blocks(inode);
 	handle = ext4_journal_start(inode, credits);

fs/ext4/file.c

@@ -55,7 +55,7 @@ static int ext4_release_file(struct inode *inode, struct file *filp)
 	return 0;
 }
 
-static void ext4_unwritten_wait(struct inode *inode)
+void ext4_unwritten_wait(struct inode *inode)
 {
 	wait_queue_head_t *wq = ext4_ioend_wq(inode);
 

fs/ext4/fsync.c

@@ -138,7 +138,7 @@ int ext4_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
 	if (inode->i_sb->s_flags & MS_RDONLY)
 		goto out;
 
-	ret = ext4_flush_completed_IO(inode);
+	ret = ext4_flush_unwritten_io(inode);
 	if (ret < 0)
 		goto out;
 

fs/ext4/indirect.c

@@ -807,9 +807,11 @@ ssize_t ext4_ind_direct_IO(int rw, struct kiocb *iocb,
 
 retry:
 	if (rw == READ && ext4_should_dioread_nolock(inode)) {
-		if (unlikely(!list_empty(&ei->i_completed_io_list)))
-			ext4_flush_completed_IO(inode);
-
+		if (unlikely(atomic_read(&EXT4_I(inode)->i_unwritten))) {
+			mutex_lock(&inode->i_mutex);
+			ext4_flush_unwritten_io(inode);
+			mutex_unlock(&inode->i_mutex);
+		}
 		/*
 		 * Nolock dioread optimization may be dynamically disabled
 		 * via ext4_inode_block_unlocked_dio(). Check inode's state

fs/ext4/page-io.c

@@ -189,8 +189,6 @@ static int ext4_do_flush_completed_IO(struct inode *inode,
 
 		list_add_tail(&io->list, &complete);
 	}
-	/* It is important to update all flags for all end_io in one shot w/o
-	 * dropping the lock.*/
 	spin_lock_irqsave(&ei->i_completed_io_lock, flags);
 	while (!list_empty(&complete)) {
 		io = list_entry(complete.next, ext4_io_end_t, list);
@@ -228,9 +226,14 @@ static void ext4_end_io_work(struct work_struct *work)
 	ext4_do_flush_completed_IO(io->inode, io);
 }
 
-int ext4_flush_completed_IO(struct inode *inode)
+int ext4_flush_unwritten_io(struct inode *inode)
 {
-	return ext4_do_flush_completed_IO(inode, NULL);
+	int ret;
+	WARN_ON_ONCE(!mutex_is_locked(&inode->i_mutex) &&
+		     !(inode->i_state & I_FREEING));
+	ret = ext4_do_flush_completed_IO(inode, NULL);
+	ext4_unwritten_wait(inode);
+	return ret;
 }
 
 ext4_io_end_t *ext4_init_io_end(struct inode *inode, gfp_t flags)