From bbd32754acbd3d7368c403e8d386e55ecb8034e7 Mon Sep 17 00:00:00 2001 From: Dave Chinner Date: Fri, 9 May 2014 14:50:12 +1000 Subject: [PATCH] repair: don't unlock prefetch tree to read discontig buffers 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 Signed-off-by: Dave Chinner Reviewed-by: Eric Sandeen Signed-off-by: Dave Chinner --- repair/prefetch.c | 51 +++++++++++++++++++++++------------------------ 1 file changed, 25 insertions(+), 26 deletions(-) diff --git a/repair/prefetch.c b/repair/prefetch.c index 65fedf564..4595310a2 100644 --- a/repair/prefetch.c +++ b/repair/prefetch.c @@ -443,27 +443,6 @@ pf_read_inode_dirs( XFS_BUF_SET_PRIORITY(bp, B_DIR_INODE); } -/* - * 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. */ @@ -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 +555,20 @@ 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 above loop + * guarantees that only the last buffer in the list will be a + * discontiguous buffer. + */ + 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 -- 2.47.2