[PATCH 3.16 216/366] ext4: Fix WARN_ON_ONCE in ext4_commit_super()

Ben Hutchings
2018-11-11T20:00:14Z

From: "Pranay Kr. Srivastava" <pranjas@gmail.com>

commit 4743f83990614af6adb09ea7aa3c37b78c4031ab upstream.

If there are racing calls to ext4_commit_super() it's possible for
another writeback of the superblock to result in the buffer being
marked with an error after we check if the buffer is marked as having
a write error and the buffer up-to-date flag is set again. If that
happens mark_buffer_dirty() can end up throwing a WARN_ON_ONCE.

Fix this by moving this check to write before we call
write_buffer_dirty(), and keeping the buffer locked during this whole
sequence.

Signed-off-by: Pranay Kr. Srivastava <pranjas@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/ext4/super.c | 30 ++++++++++++++++--------------
1 file changed, 16 insertions(+), 14 deletions(-)

--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -4653,20 +4653,6 @@ static int ext4_commit_super(struct supe

if (!sbh || block_device_ejected(sb))
return error;
- if (buffer_write_io_error(sbh)) {
- /*
- * Oh, dear. A previous attempt to write the
- * superblock failed. This could happen because the
- * USB device was yanked out. Or it could happen to
- * be a transient write error and maybe the block will
- * be remapped. Nothing we can do but to retry the
- * write and hope for the best.
- */
- ext4_msg(sb, KERN_ERR, "previous I/O error to "
- "superblock detected");
- clear_buffer_write_io_error(sbh);
- set_buffer_uptodate(sbh);
- }
/*
* If the file system is mounted read-only, don't update the
* superblock write time. This avoids updating the superblock
@@ -4695,7 +4681,23 @@ static int ext4_commit_super(struct supe
&EXT4_SB(sb)->s_freeinodes_counter));
BUFFER_TRACE(sbh, "marking dirty");
ext4_superblock_csum_set(sb);
+ lock_buffer(sbh);
+ if (buffer_write_io_error(sbh)) {
+ /*
+ * Oh, dear. A previous attempt to write the
+ * superblock failed. This could happen because the
+ * USB device was yanked out. Or it could happen to
+ * be a transient write error and maybe the block will
+ * be remapped. Nothing we can do but to retry the
+ * write and hope for the best.
+ */
+ ext4_msg(sb, KERN_ERR, "previous I/O error to "
+ "superblock detected");
+ clear_buffer_write_io_error(sbh);
+ set_buffer_uptodate(sbh);
+ }
mark_buffer_dirty(sbh);
+ unlock_buffer(sbh);
if (sync) {
error = sync_dirty_buffer(sbh);
if (error)