Re: [PATCH] xfs: don't free EFIs before the EFDs are committed

Mark Tinguely
2013-04-03T19:12:09Z

On 04/02/13 22:09, Dave Chinner wrote:
> From: Dave Chinner<dchinner@redhat.com>
>
> Filesystems are occasionally being shut down with this error:
>
> xfs_trans_ail_delete_bulk: attempting to delete a log item that is
> not in the AIL.
>
> It was diagnosed to be related to the EFI/EFD commit order when the
> EFI and EFD are in different checkpoints and the EFD is committed
> before the EFI here:
>
> http://oss.sgi.com/archives/xfs/2013-01/msg00082.html
>
> The real problem is that a single bit cannot fully describe the
> states that the EFI/EFD processing can be in. These completion
> states are:
>
> EFI EFI in AIL EFD Result
> committed/unpinned Yes committed OK
> committed/pinned No committed Shutdown
> uncommitted No committed Shutdown
>
>
> Note that the "result" field is what should happen, not what does
> happen. The current logic is broken and handles the first two cases
> correctly by luck. That is, the code will free the EFI if the
> XFS_EFI_COMMITTED bit is*not* set, rather than if it is set. The
> inverted logic "works" because if both EFI and EFD are committed,
> then the first __xfs_efi_release() call clears the XFS_EFI_COMMITTED
> bit, and the second frees the EFI item. Hence as long as
> xfs_efi_item_committed() has been called, everything appears to be
> fine.
>
> It is the third case where the logic fails - where
> xfs_efd_item_committed() is called before xfs_efi_item_committed(),
> and that results in the EFI being freed before it has been
> committed. That is the bug that triggered the shutdown, d hence
> keeping track of whether the EFI has been committed or not is
> insufficient to correctly order the EFI/EFD operations w.r.t. the
> AIL.
>

I think the exist xfs_efi routines are working correctly.

the xfs_trans_committed_bulk() for the efi does the IOP_COMMITTED (sets
the XFS_EFI_COMMITTED bit), add the entry to the AIL and then an
IOP_UNPIN() which clears the XFS_EFI_COMMITTED since it is set, it
should not release the efi and remove the entry from the AIL. It also
correctly detects if the EFD is committed before the EFI by shutting
down the filesystem.

This patch seems to paper over why the EFD will come before the EFI.

The CIL commits are in the correct order - protected by the xlog_wait()
but the callbacks are out of order.

My previous patch fixed the callback from happening out of sequence.

--Mark.