[PATCH 3.16 079/133] xfs: fix incorrect log_flushed on fsync

From: Ben Hutchings
Date: Tue Nov 21 2017 - 21:31:24 EST

3.16.51-rc1 review patch. If anyone has any objections, please let me know.


From: Amir Goldstein <amir73il@xxxxxxxxx>

commit 47c7d0b19502583120c3f396c7559e7a77288a68 upstream.

When calling into _xfs_log_force{,_lsn}() with a pointer
to log_flushed variable, log_flushed will be set to 1 if:
1. xlog_sync() is called to flush the active log buffer
2. xlog_wait() is called to wait on a syncing log buffers

xfs_file_fsync() checks the value of log_flushed after
_xfs_log_force_lsn() call to optimize away an explicit
PREFLUSH request to the data block device after writing
out all the file's pages to disk.

This optimization is incorrect in the following sequence of events:

Task A Task B
[submit PREFLUSH]
[submit WRITE X]
[endio WRITE X]
[endio PREFLUSH]

The write X is not guarantied to be on persistent storage
when PREFLUSH request in completed, because write A was submitted
after the PREFLUSH request, but xfs_file_fsync() of task A will
be notified of log_flushed=1 and will skip explicit flush.

If the system crashes after fsync of task A, write X may not be
present on disk after reboot.

This bug was discovered and demonstrated using Josef Bacik's
dm-log-writes target, which can be used to record block io operations
and then replay a subset of these operations onto the target device.
The test goes something like this:
- Use fsx to execute ops of a file and record ops on log device
- Every now and then fsync the file, store md5 of file and mark
the location in the log
- Then replay log onto device for each mark, mount fs and compare
md5 of file to stored value

Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Josef Bacik <jbacik@xxxxxx>
Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
fs/xfs/xfs_log.c | 7 -------
1 file changed, 7 deletions(-)

--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -3257,8 +3257,6 @@ maybe_sleep:
if (iclog->ic_state & XLOG_STATE_IOERROR)
return XFS_ERROR(EIO);
- if (log_flushed)
- *log_flushed = 1;
} else {

@@ -3366,8 +3364,6 @@ try_again:

- if (log_flushed)
- *log_flushed = 1;
already_slept = 1;
goto try_again;
@@ -3401,9 +3397,6 @@ try_again:
if (iclog->ic_state & XLOG_STATE_IOERROR)
return XFS_ERROR(EIO);
- if (log_flushed)
- *log_flushed = 1;
} else { /* just return */