xfstests generic/388,475 occasionally reproduce assertion failures
in xfs_buf_item_unpin() when the final bli reference is dropped on
an invalidated buffer and the buffer is not locked as it is expected
to be. Invalidated buffers should remain locked on transaction
commit until the final unpin, at which point the buffer is removed
from the AIL and the bli is freed since stale buffers are not
written back.
The assert failures are associated with filesystem shutdown,
typically due to log I/O errors injected by the test. The
problematic situation can occur if the shutdown happens to cause a
race between an active transaction that has invalidated a particular
buffer and an I/O error on a log buffer that contains the bli
associated with the same (now stale) buffer.
Both transaction and log contexts acquire a bli reference. If the
transaction has already invalidated the buffer by the time the I/O
error occurs and ends up aborting due to shutdown, the transaction
and log hold the last two references to a stale bli. If the
transaction cancel occurs first, it treats the buffer as non-stale
due to the aborted state: the bli reference is dropped and the
buffer is released/unlocked. The log buffer I/O error handling
eventually calls into xfs_buf_item_unpin(), drops the final
reference to the bli and treats it as stale. The buffer wasn't left
locked by xfs_buf_item_unlock(), however, so the assert fails and
the buffer is double unlocked. The latter problem is mitigated by
the fact that the fs is shutdown and no further damage is possible.
->iop_unlock() of an invalidated buffer should behave consistently
with respect to the bli refcount, regardless of aborted state. If
the refcount remains elevated on commit, we know the bli is awaiting
an unpin (since it can't be in another transaction) and will be
handled appropriately on log buffer completion. If the final bli
reference of an invalidated buffer is dropped in ->iop_unlock(), we
can assume the transaction has aborted because invalidation implies
a dirty transaction. In the non-abort case, the log would have
acquired a bli reference in ->iop_pin() and prevented bli release at
->iop_unlock() time. In the abort case the item must be freed and
buffer unlocked because it wasn't pinned by the log.
Rework xfs_buf_item_unlock() to simplify the currently circuitous
and duplicate logic and leave invalidated buffers locked based on
bli refcount, regardless of aborted state. This ensures that a
pinned, stale buffer is always found locked when eventually
unpinned.
Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>
---
Hi all,
This survives the assert reproducer, several xfstests runs with v4/v5
superblock filesystems, and fsstress and fsx workloads over a few hours.
Brian
fs/xfs/xfs_buf_item.c | 85 +++++++++++++++++++------------------------
1 file changed, 37 insertions(+), 48 deletions(-)
diff --git a/fs/xfs/xfs_buf_item.c b/fs/xfs/xfs_buf_item.c
index 1c9d1398980b..5e8b91543d93 100644
--- a/fs/xfs/xfs_buf_item.c
+++ b/fs/xfs/xfs_buf_item.c
@@ -556,73 +556,62 @@ xfs_buf_item_unlock(
{
struct xfs_buf_log_item *bip = BUF_ITEM(lip);
struct xfs_buf *bp = bip->bli_buf;
+ int freed;
bool aborted;
bool hold = !!(bip->bli_flags & XFS_BLI_HOLD);
bool dirty = !!(bip->bli_flags & XFS_BLI_DIRTY);
+ bool stale = !!(bip->bli_flags & XFS_BLI_STALE);
#if defined(DEBUG) || defined(XFS_WARN)
bool ordered = !!(bip->bli_flags & XFS_BLI_ORDERED);
#endif
+ /*
+ * The bli dirty state should match whether the blf has logged segments
+ * except for ordered buffers, where only the bli should be dirty.
+ */
+ ASSERT((!ordered && dirty == xfs_buf_item_dirty_format(bip)) ||
+ (ordered && dirty && !xfs_buf_item_dirty_format(bip)));
+ ASSERT(!stale || (bip->__bli_format.blf_flags & XFS_BLF_CANCEL));
- aborted = test_bit(XFS_LI_ABORTED, &lip->li_flags);
+ trace_xfs_buf_item_unlock(bip);
- /* Clear the buffer's association with this transaction. */
- bp->b_transp = NULL;
+ aborted = test_bit(XFS_LI_ABORTED, &lip->li_flags);
/*
- * The per-transaction state has been copied above so clear it from the
- * bli.
+ * Clear the buffer's association with this transaction and
+ * per-transaction state from the bli, which has been copied above.
*/
+ bp->b_transp = NULL;
bip->bli_flags &= ~(XFS_BLI_LOGGED | XFS_BLI_HOLD | XFS_BLI_ORDERED);
/*
- * 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.
+ * Drop the transaction bli reference and free the item if clean or
+ * aborted and we had the last reference. In either case this is the
+ * last opportunity to free the item since it won't be written back.
+ * Otherwise, the bli is still referenced or dirty and will be freed on
+ * final unpin of the buffer (if stale) or writeback completion.
*/
- if (bip->bli_flags & XFS_BLI_STALE) {
- trace_xfs_buf_item_unlock_stale(bip);
- ASSERT(bip->__bli_format.blf_flags & XFS_BLF_CANCEL);
- if (!aborted) {
- atomic_dec(&bip->bli_refcount);
- return;
- }
+ freed = atomic_dec_and_test(&bip->bli_refcount);
+ if (freed && (aborted || !dirty)) {
+ ASSERT(!aborted || XFS_FORCED_SHUTDOWN(lip->li_mountp));
+ ASSERT(!stale || aborted);
+ /* an aborted item may be in the AIL, remove it first */
+ if (aborted)
+ xfs_trans_ail_remove(lip, SHUTDOWN_LOG_IO_ERROR);
+ xfs_buf_item_relse(bp);
}
- trace_xfs_buf_item_unlock(bip);
-
- /*
- * If the buf item isn't tracking any data, free it, otherwise drop the
- * reference we hold to it. If we are aborting the transaction, this may
- * be the only reference to the buf item, so we free it anyway
- * regardless of whether it is dirty or not. A dirty abort implies a
- * shutdown, anyway.
- *
- * The bli dirty state should match whether the blf has logged segments
- * except for ordered buffers, where only the bli should be dirty.
- */
- ASSERT((!ordered && dirty == xfs_buf_item_dirty_format(bip)) ||
- (ordered && dirty && !xfs_buf_item_dirty_format(bip)));
-
/*
- * Clean buffers, by definition, cannot be in the AIL. However, aborted
- * buffers may be in the AIL regardless of dirty state. An aborted
- * transaction that invalidates a buffer already in the AIL may have
- * marked it stale and cleared the dirty state, for example.
- *
- * Therefore if we are aborting a buffer and we've just taken the last
- * reference away, we have to check if it is in the AIL before freeing
- * it. We need to free it in this case, because an aborted transaction
- * has already shut the filesystem down and this is the last chance we
- * will have to do so.
+ * If the buffer was invalidated, leave it locked on transaction commit
+ * unless we just dropped the final reference. The latter case should
+ * only ever happen on abort because invalidation dirties the
+ * transaction and the log would have grabbed another bli reference when
+ * the buffer was pinned. In the non-abort case, the buffer is unlocked
+ * on final unpin and the bli freed since stale buffers are not written
+ * back.
*/
- if (atomic_dec_and_test(&bip->bli_refcount)) {
- if (aborted) {
- ASSERT(XFS_FORCED_SHUTDOWN(lip->li_mountp));
- xfs_trans_ail_remove(lip, SHUTDOWN_LOG_IO_ERROR);
- xfs_buf_item_relse(bp);
- } else if (!dirty)
- xfs_buf_item_relse(bp);
- }
+ if (stale && !freed)
+ return;
+ ASSERT(!stale || (aborted && freed));
if (!hold)
xfs_buf_relse(bp);
--
2.17.1
