Re: [2.6.27-rc4] XFS i_lock vs i_iolock...

[Dave Chinner <david@fromorbit.com>]

On Mon, Aug 25, 2008 at 11:55:32PM +0200, Christoph Hellwig wrote:
> On Mon, Aug 25, 2008 at 08:59:33AM +0200, Peter Zijlstra wrote:
> > How can you take two locks in one go? It seems to me you always need to
> > take them one after another, and as soon as you do that, you have
> > ordering constraints.
> 
> Yes, you would.  Except that in all other places we only have a single
> iolock involved, so the ordering of the second iolock and second ilock
> don't matter.
> 
> Because of that I think declaring that xfs_lock_two_inodes can just
> lock on lock type at a time might be the better solution.

Agreed. Patch below.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

XFS: prevent lockdep false positives when locking two inodes

If we call xfs_lock_two_inodes() to grab both the iolock and
the ilock, then drop the ilocks on both inodes, then grab
them again (as xfs_swap_extents() does) then lockdep will
report a locking order problem. This is a false positive.

To avoid this, disallow xfs_lock_two_inodes() fom locking both
inode locks at once - force calers to make two separate calls.
This means that nested dropping and regaining of the ilocks
will retain the same lockdep subclass and so lockdep will
not see anything wrong with this code.

Signed-off-by: Dave Chinner <david@fromorbit.com>
---
 fs/xfs/xfs_dfrag.c    |    9 ++++++++-
 fs/xfs/xfs_vnodeops.c |   10 ++++++++++
 2 files changed, 18 insertions(+), 1 deletions(-)

diff --git a/fs/xfs/xfs_dfrag.c b/fs/xfs/xfs_dfrag.c
index 760f4c5..75b0cd4 100644
--- a/fs/xfs/xfs_dfrag.c
+++ b/fs/xfs/xfs_dfrag.c
@@ -149,7 +149,14 @@ xfs_swap_extents(
 
 	sbp = &sxp->sx_stat;
 
-	xfs_lock_two_inodes(ip, tip, lock_flags);
+	/*
+	 * 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.
+	 */
+	xfs_lock_two_inodes(ip, tip, XFS_IOLOCK_EXCL);
+	xfs_lock_two_inodes(ip, tip, XFS_ILOCK_EXCL);
 	locked = 1;
 
 	/* Verify that both files have the same format */
diff --git a/fs/xfs/xfs_vnodeops.c b/fs/xfs/xfs_vnodeops.c
index f108102..cb1b5fd 100644
--- a/fs/xfs/xfs_vnodeops.c
+++ b/fs/xfs/xfs_vnodeops.c
@@ -1836,6 +1836,12 @@ again:
 #endif
 }
 
+/*
+ * xfs_lock_two_inodes() can only be used to lock one type of lock
+ * at a time - the iolock or the ilock, but not both at once. If
+ * we lock both at once, lockdep will report false positives saying
+ * we have violated locking orders.
+ */
 void
 xfs_lock_two_inodes(
 	xfs_inode_t		*ip0,
@@ -1846,7 +1852,11 @@ xfs_lock_two_inodes(
 	int			attempts = 0;
 	xfs_log_item_t		*lp;
 
+#ifdef DEBUG
+	if (lock_mode & (XFS_IOLOCK_SHARED|XFS_IOLOCK_EXCL))
+		ASSERT((lock_mode & (XFS_ILOCK_SHARED|XFS_ILOCK_EXCL)) == 0);
 	ASSERT(ip0->i_ino != ip1->i_ino);
+#endif
 
 	if (ip0->i_ino > ip1->i_ino) {
 		temp = ip0;