Re: [PATCH 03/12] xfs: modify buffer item reference counting

Alex Elder
2010-05-19T18:33:43Z

On Tue, 2010-05-18 at 09:24 +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> The buffer log item reference counts used to take referenceѕ for every
> transaction, similar to the pin counting. This is symmetric (like the
> pin/unpin) with respect to transaction completion, but with dleayed logging
> becomes assymetric as the pinning becomes assymetric w.r.t. transaction
> completion.

Doing the reference counts this way is an improvement.

I have a question below.

. . .
> --- a/fs/xfs/xfs_buf_item.c
> +++ b/fs/xfs/xfs_buf_item.c
. . .
> STATIC void
> xfs_buf_item_unlock(
> @@ -514,73 +523,54 @@ xfs_buf_item_unlock(
>
> bp = bip->bli_buf;
>
> - /*
> - * Clear the buffer's association with this transaction.
> - */
> + /* Clear the buffer's association with this transaction. */
> XFS_BUF_SET_FSPRIVATE2(bp, NULL);
>
> /*
> - * If this is a transaction abort, don't return early.
> - * Instead, allow the brelse to happen.
> - * Normally it would be done for stale (cancelled) buffers
> - * at unpin time, but we'll never go through the pin/unpin
> - * cycle if we abort inside commit.
> + * If this is a transaction abort, don't return early. Instead, allow
> + * the brelse to happen. Normally it would be done for stale
> + * (cancelled) buffers at unpin time, but we'll never go through the
> + * pin/unpin cycle if we abort inside commit.
> */
> aborted = (bip->bli_item.li_flags & XFS_LI_ABORTED) != 0;
>
> /*
> - * If the buf item is marked stale, then don't do anything.
> - * We'll unlock the buffer and free the buf item when the
> - * buffer is unpinned for the last time.
> + * Before possibly freeing the buf item, determine if we should
> + * release the buffer at the end of this routine.
> + */
> + hold = bip->bli_flags & XFS_BLI_HOLD;
> +
> + /* Clear the per transaction state. */
> + bip->bli_flags &= ~(XFS_BLI_LOGGED | XFS_BLI_HOLD);
> +
> + /*
> + * If the buf item is marked stale, then don't do anything. We'll
> + * unlock the buffer and free the buf item when the buffer is unpinned
> + * for the last time.
> */
> if (bip->bli_flags & XFS_BLI_STALE) {
> - bip->bli_flags &= ~XFS_BLI_LOGGED;
> trace_xfs_buf_item_unlock_stale(bip);
> ASSERT(bip->bli_format.blf_flags & XFS_BLI_CANCEL);
> - if (!aborted)
> + if (!aborted) {
> + atomic_dec(&bip->bli_refcount);

I notice that, unlike before, if you return via this path
the XFS_BLI_HOLD flag will have been turned off. I guess
I don't know off hand whether this is an issue. Can you
explain why it is not?

> return;
> + }
> }
>
. . .