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

Mark Tinguely
2013-04-04T22:06:32Z

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, and 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.
>
> What we really want is this: the EFI is always placed into the
> AIL before the last reference goes away. The only way to guarantee
> that is that the EFI is not freed until after it has been unpinned
> *and* the EFD has been committed. That is, restructure the logic so
> that the only case that can occur is the first case.
>
> This can be done easily by replacing the XFS_EFI_COMMITTED with an
> EFI reference count. The EFI is initialised with it's own count, and
> that is not released until it is unpinned. However, there is a
> complication to this method - the high level EFI/EFD code in
> xfs_bmap_finish() does not hold direct references to the EFI
> structure, and runs a transaction commit between the EFI and EFD
> processing. Hence the EFI can be freed even before the EFD is
> created using such a method.
>
> Further, log recovery uses the AIL for tracking EFI/EFDs that need
> to be recovered, but it uses the AIL *differently* to the EFI
> transaction commit. Hence log recovery never pins or unpins EFIs, so
> we can't drop the EFI reference count indirectly to free the EFI.
>
> However, this doesn't prevent us from using a reference count here.
> There is a 1:1 relationship between EFIs and EFDs, so when we
> initialise the EFI we can take a reference count for the EFD as
> well. This solves the xfs_bmap_finish() issue - the EFI will never
> be freed until the EFD is processed. In terms of log recovery,
> during the committing of the EFD we can look for the
> XFS_EFI_RECOVERED bit being set and drop the EFI reference as well,
> thereby ensuring everything works correctly there as well.

Duh me, beside what our problem where the cil push callbacks are called
out of order, there can be several xfs_trans_committed_bulk() running at
the same time.

Yep, the counter allows efi and efd to happen in any order and make sure
the efi is entered into the AIL.

Okay, the pass2 of recovery places the efi item into the AIL, and the
second decrement on the counter when the XFS_EFI_RECOVERED bit gets the
counter to zero in the recovery case because there is no unpin.

Reviewed-by: Mark Tinguely <tinguely@sgi.com>

This patch also prevents the only chance of detecting of out of sequence
ctx callbacks. The out of sequence ctx callbacks *does* matter because
the li_lsn is set by the ctx->start_lsn and the lowest li_lsn is used to
move the tail. When the callbacks are out of order, it is possible for
the larger lsn of the greater ctx sequence to move the log tail beyond a
lower ctx sequence entries before the entries for the lower sequence ctx
are moved into the AIL.

--Mark.