Re: [PATCH 3/9] xfs: rework xfs_buf_bio_endio error handling

Brian Foster
2014-08-15T13:18:42Z

On Fri, Aug 15, 2014 at 04:39:01PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> Currently the report of a bio error from completion
> immediately marks the buffer with an error. The issue is that this
> is racy w.r.t. synchronous IO - the submitter can see b_error being
> set before the IO is complete, and hence we cannot differentiate
> between submission failures and completion failures.
>
> Add an internal b_io_error field protected by the b_lock to catch IO
> completion errors, and only propagate that to the buffer during
> final IO completion handling. Hence we can tell in xfs_buf_iorequest
> if we've had a submission failure bey checking bp->b_error before
> dropping our b_io_remaining reference - that reference will prevent
> b_io_error values from being propagated to b_error in the event that
> completion races with submission.
>
> In doing so, allow xfs_buf_iorequest to return an error. That way,
> the caller can check for submission errors safely if required, and
> easily distinguish them from completion errors that come from
> xfs_buf_iowait().
>

I don't see any changes to xfs_buf_iorequest() at all in this one. That
aside, it looks fine. It's not how I was thinking about it, but a clear
separation of submission error regardless.

Brian

> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/xfs_buf.c | 18 ++++++++++++++++--
> fs/xfs/xfs_buf.h | 1 +
> 2 files changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index 1b7f0bc..58ae34c 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -1008,6 +1008,13 @@ xfs_buf_ioend(
>
> bp->b_flags &= ~(XBF_READ | XBF_WRITE | XBF_READ_AHEAD);
>
> + /*
> + * Pull in IO completion errors now. We are guaranteed to be running
> + * single threaded, so we don't need the lock to read b_io_error.
> + */
> + if (!bp->b_error && bp->b_io_error)
> + xfs_buf_ioerror(bp, bp->b_io_error);
> +
> if (!bp->b_error) {
> bp->b_flags |= XBF_DONE;
>
> @@ -1186,8 +1193,12 @@ xfs_buf_bio_end_io(
> * don't overwrite existing errors - otherwise we can lose errors on
> * buffers that require multiple bios to complete.
> */
> - if (!bp->b_error)
> - xfs_buf_ioerror(bp, error);
> + if (error) {
> + spin_lock(&bp->b_lock);
> + if (!bp->b_io_error)
> + bp->b_io_error = error;
> + spin_unlock(&bp->b_lock);
> + }
>
> if (!bp->b_error && xfs_buf_is_vmapped(bp) && (bp->b_flags & XBF_READ))
> invalidate_kernel_vmap_range(bp->b_addr, xfs_buf_vmap_len(bp));
> @@ -1373,6 +1384,9 @@ xfs_buf_iorequest(
> if (bp->b_flags & XBF_WRITE)
> xfs_buf_wait_unpin(bp);
>
> + /* clear the internal error state to avoid spurious errors */
> + bp->b_io_error = 0;
> +
> /*
> * Take references to the buffer. For XBF_ASYNC buffers, holding a
> * reference for as long as submission takes is all that is necessary
> diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
> index 4585c15..44db8cd 100644
> --- a/fs/xfs/xfs_buf.h
> +++ b/fs/xfs/xfs_buf.h
> @@ -158,6 +158,7 @@ typedef struct xfs_buf {
> struct list_head b_lru; /* lru list */
> spinlock_t b_lock; /* internal state lock */
> unsigned int b_state; /* internal state flags */
> + int b_io_error; /* internal IO error state */
> wait_queue_head_t b_waiters; /* unpin waiters */
> struct list_head b_list;
> struct xfs_perag *b_pag; /* contains rbtree root */
> --
> 2.0.0
>
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