ext4: flush any pending end_io requests before DIO reads w/dioread_nolock

ext4: flush any pending end_io requests before DIO reads w/dioread_nolock

commit dccaf33fa37a1bc5d651baeb3bfeb6becb86597b upstream.

(backported to 3.0 by mjt)

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>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>