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

Dave Chinner
2014-08-15T06:39:15Z

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().

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