[PATCH 1/2] xfs: fix swapext ilock deadlock

Dave Chinner
2014-07-31T06:12:21Z

From: Dave Chinner <dchinner@redhat.com>

xfs_swap_extents() holds the ilock over a call to
filemap_write_and_wait(), which can then try to write data and take
the ilock. That causes a self-deadlock.

Fix the deadlock and clean up the code by separating the locking
appropriately. Add a lockflags variable to track what locks we are
holding as we gain and drop them and cleanup the error handling to
always use "out_unlock" with the lockflags variable.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
fs/xfs/xfs_bmap_util.c | 33 ++++++++++++++++++---------------
1 file changed, 18 insertions(+), 15 deletions(-)

diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
index bbc748a..3c60c43 100644
--- a/fs/xfs/xfs_bmap_util.c
+++ b/fs/xfs/xfs_bmap_util.c
@@ -1633,6 +1633,7 @@ xfs_swap_extents(
int aforkblks = 0;
int taforkblks = 0;
__uint64_t tmp;
+ int lock_flags;

tempifp = kmem_alloc(sizeof(xfs_ifork_t), KM_MAYFAIL);
if (!tempifp) {
@@ -1641,13 +1642,13 @@ xfs_swap_extents(
}

/*
- * we have to do two separate lock calls here to keep lockdep
- * happy. If we try to get all the locks in one call, lock will
- * report false positives when we drop the ILOCK and regain them
- * below.
+ * Lock up the inodes against other IO and truncate to begin with.
+ * Then we can ensure the inodes are flushed and have no page cache
+ * safely. Once we have done this we can take the ilocks and do the rest
+ * of the checks.
*/
+ lock_flags = XFS_IOLOCK_EXCL;
xfs_lock_two_inodes(ip, tip, XFS_IOLOCK_EXCL);
- xfs_lock_two_inodes(ip, tip, XFS_ILOCK_EXCL);

/* Verify that both files have the same format */
if ((ip->i_d.di_mode & S_IFMT) != (tip->i_d.di_mode & S_IFMT)) {
@@ -1666,6 +1667,9 @@ xfs_swap_extents(
goto out_unlock;
truncate_pagecache_range(VFS_I(tip), 0, -1);

+ xfs_lock_two_inodes(ip, tip, XFS_ILOCK_EXCL);
+ lock_flags |= XFS_ILOCK_EXCL;
+
/* Verify O_DIRECT for ftmp */
if (VFS_I(tip)->i_mapping->nrpages) {
error = -EINVAL;
@@ -1720,6 +1724,7 @@ xfs_swap_extents(

xfs_iunlock(ip, XFS_ILOCK_EXCL);
xfs_iunlock(tip, XFS_ILOCK_EXCL);
+ lock_flags &= ~XFS_ILOCK_EXCL;

/*
* There is a race condition here since we gave up the
@@ -1732,13 +1737,11 @@ xfs_swap_extents(

tp = xfs_trans_alloc(mp, XFS_TRANS_SWAPEXT);
error = xfs_trans_reserve(tp, &M_RES(mp)->tr_ichange, 0, 0);
- if (error) {
- xfs_iunlock(ip, XFS_IOLOCK_EXCL);
- xfs_iunlock(tip, XFS_IOLOCK_EXCL);
- xfs_trans_cancel(tp, 0);
- goto out;
- }
+ if (error)
+ goto out_trans_cancel;
+
xfs_lock_two_inodes(ip, tip, XFS_ILOCK_EXCL);
+ lock_flags |= XFS_ILOCK_EXCL;

/*
* Count the number of extended attribute blocks
@@ -1757,8 +1760,8 @@ xfs_swap_extents(
goto out_trans_cancel;
}

- xfs_trans_ijoin(tp, ip, XFS_ILOCK_EXCL | XFS_IOLOCK_EXCL);
- xfs_trans_ijoin(tp, tip, XFS_ILOCK_EXCL | XFS_IOLOCK_EXCL);
+ xfs_trans_ijoin(tp, ip, lock_flags);
+ xfs_trans_ijoin(tp, tip, lock_flags);

/*
* Before we've swapped the forks, lets set the owners of the forks
@@ -1887,8 +1890,8 @@ out:
return error;

out_unlock:
- xfs_iunlock(ip, XFS_ILOCK_EXCL | XFS_IOLOCK_EXCL);
- xfs_iunlock(tip, XFS_ILOCK_EXCL | XFS_IOLOCK_EXCL);
+ xfs_iunlock(ip, lock_flags);
+ xfs_iunlock(tip, lock_flags);
goto out;

out_trans_cancel:
--
2.0.0