> 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;
