(backported to 3.0 by mjt)

commit dccaf33fa37a1bc5d651baeb3bfeb6becb86597b
Author: Jiaying Zhang <jiayingz@google.com>
Date:   Fri Aug 19 19:13:32 2011 -0400

    ext4: flush any pending end_io requests before DIO reads w/dioread_nolock
    
    There is a race between ext4 buffer write and direct_IO read with
    dioread_nolock mount option enabled. The problem is that we clear
    PageWriteback flag during end_io time but will do
    uninitialized-to-initialized extent conversion later with dioread_nolock.
    If an O_direct read request comes in during this period, ext4 will return
    zero instead of the recently written data.
    
    This patch checks whether there are any pending uninitialized-to-initialized
    extent conversion requests before doing O_direct read to close the race.
    Note that this is just a bandaid fix. The fundamental issue is that we
    clear PageWriteback flag before we really complete an IO, which is
    problem-prone. To fix the fundamental issue, we may need to implement an
    extent tree cache that we can use to look up pending to-be-converted extents.
    
    Signed-off-by: Jiaying Zhang <jiayingz@google.com>
    Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
    Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
    Cc: stable@kernel.org

diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
index b8602cd..0962642 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3507,12 +3507,17 @@ ssize_t ext4_ind_direct_IO(int rw, struct kiocb *iocb,
 	}
 
 retry:
-	if (rw == READ && ext4_should_dioread_nolock(inode))
+	if (rw == READ && ext4_should_dioread_nolock(inode)) {
+		if (unlikely(!list_empty(&ei->i_completed_io_list))) {
+			mutex_lock(&inode->i_mutex);
+			ext4_flush_completed_IO(inode);
+			mutex_unlock(&inode->i_mutex);
+		}
 		ret = __blockdev_direct_IO(rw, iocb, inode,
 				 inode->i_sb->s_bdev, iov,
 				 offset, nr_segs,
 				 ext4_get_block, NULL, NULL, 0);
-	else {
+	} else {
 		ret = blockdev_direct_IO(rw, iocb, inode,
				 inode->i_sb->s_bdev, iov,
 				 offset, nr_segs,