[PATCH 1/2] repair: don't unlock prefetch tree to read discontig buffers

[Dave Chinner <david@fromorbit.com>]

From: Dave Chinner <dchinner@redhat.com>

The way discontiguous buffers are currently handled in prefetch is
by unlocking the prefetch tree and reading them one at a time in
pf_read_discontig(), inside the normal loop of searching for buffers
to read in a more optimized fashion.

But by unlocking the tree, we allow other threads to come in and
find buffers which we've already stashed locally on our bplist[].
If 2 threads think they own the same set of buffers, they may both
try to delete them from the prefetch btree, and the second one to
arrive will not find it, resulting in:

        fatal error -- prefetch corruption

To fix this, simply abort the buffer gathering loop when we come
across a discontiguous buffer, process the gathered list as per
normal, and then after running the large optimised read, check to
see if the last buffer on the list is a discontiguous buffer.
If is is discontiguous, then issue the discontiguous buffer read
while the locks are not held. We only ever have one discontiguous
buffer per read loop, so it is safe just to check the last buffer in
the list.

The fix is loosely based on a a patch provided by Eric Sandeen, who
did all the hard work of finding the bug and demonstrating how to
fix it.

Reported-by:Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
 repair/prefetch.c | 53 +++++++++++++++++++++++++++--------------------------
 1 file changed, 27 insertions(+), 26 deletions(-)

diff --git a/repair/prefetch.c b/repair/prefetch.c
index 65fedf5..e269f1f 100644
--- a/repair/prefetch.c
+++ b/repair/prefetch.c
@@ -444,27 +444,6 @@ pf_read_inode_dirs(
 }
 
 /*
- * Discontiguous buffers require multiple IOs to fill, so we can't use any
- * linearising, hole filling algorithms on them to avoid seeks. Just remove them
- * for the prefetch queue and read them straight into the cache and release
- * them.
- */
-static void
-pf_read_discontig(
-	struct prefetch_args	*args,
-	struct xfs_buf		*bp)
-{
-	if (!btree_delete(args->io_queue, XFS_DADDR_TO_FSB(mp, bp->b_bn)))
-		do_error(_("prefetch corruption\n"));
-
-	pthread_mutex_unlock(&args->lock);
-	libxfs_readbufr_map(mp->m_ddev_targp, bp, 0);
-	bp->b_flags |= LIBXFS_B_UNCHECKED;
-	libxfs_putbuf(bp);
-	pthread_mutex_lock(&args->lock);
-}
-
-/*
  * pf_batch_read must be called with the lock locked.
  */
 static void
@@ -496,13 +475,19 @@ pf_batch_read(
 		}
 		while (bplist[num] && num < MAX_BUFS && fsbno < max_fsbno) {
 			/*
-			 * Handle discontiguous buffers outside the seek
-			 * optimised IO loop below.
+			 * Discontiguous buffers need special handling, so stop
+			 * gathering new buffers and process the list and this
+			 * discontigous buffer immediately. This avoids the
+			 * complexity of keeping a separate discontigous buffer
+			 * list and seeking back over ranges we've already done
+			 * optimised reads for.
 			 */
 			if ((bplist[num]->b_flags & LIBXFS_B_DISCONTIG)) {
-				pf_read_discontig(args, bplist[num]);
-				bplist[num] = NULL;
-			} else if (which != PF_META_ONLY ||
+				num++;
+				break;
+			}
+
+			if (which != PF_META_ONLY ||
 				   !B_IS_INODE(XFS_BUF_PRIORITY(bplist[num])))
 				num++;
 			if (num == MAX_BUFS)
@@ -570,6 +555,22 @@ pf_batch_read(
 		 * now read the data and put into the xfs_but_t's
 		 */
 		len = pread64(mp_fd, buf, (int)(last_off - first_off), first_off);
+
+		/*
+		 * Check the last buffer on the list to see if we need to
+		 * process a discontiguous buffer. The gather loop guarantees
+		 * we only ever have a single discontig buffer on the list,
+		 * and that it is the last buffer, so it is safe to do this
+		 * check and read here like this while we aren't holding any
+		 * locks.
+		 */
+		if ((bplist[num - 1]->b_flags & LIBXFS_B_DISCONTIG)) {
+			libxfs_readbufr_map(mp->m_ddev_targp, bplist[num - 1], 0);
+			bplist[num - 1]->b_flags |= LIBXFS_B_UNCHECKED;
+			libxfs_putbuf(bplist[num - 1]);
+			num--;
+		}
+
 		if (len > 0) {
 			/*
 			 * go through the xfs_buf_t list copying from the
-- 
1.9.0

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