xfs: fix efi/efd error handling to avoid fs shutdown hangs

Freeing an extent in XFS involves logging an EFI (extent free
intention), freeing the actual extent, and logging an EFD (extent
free done). The EFI object is created with a reference count of 2:
one for the current transaction and one for the subsequently created
EFD. Under normal circumstances, the first reference is dropped when
the EFI is unpinned and the second reference is dropped when the EFD
is committed to the on-disk log.

In event of errors or filesystem shutdown, there are various
potential cleanup scenarios depending on the state of the EFI/EFD.
The cleanup scenarios are confusing and racy, as demonstrated by the
following test sequence:

	# mount $dev $mnt
	# fsstress -d $mnt -n 99999 -p 16 -z -f fallocate=1 \
		-f punch=1 -f creat=1 -f unlink=1 &
	# sleep 5
	# killall -9 fsstress; wait
	# godown -f $mnt
	# umount

... in which the final umount can hang due to the AIL being pinned
indefinitely by one or more EFI items. This can occur due to several
conditions. For example, if the shutdown occurs after the EFI is
committed to the on-disk log and the EFD committed to the CIL, but
before the EFD committed to the log, the EFD iop_committed() abort
handler does not drop its reference to the EFI. Alternatively,
manual error injection in the xfs_bmap_finish() codepath shows that
if an error occurs after the EFI transaction is committed but before
the EFD is constructed and logged, the EFI is never released from
the AIL.

Update the EFI/EFD item handling code to use a more straightforward
and reliable approach to error handling. If an error occurs after
the EFI transaction is committed and before the EFD is constructed,
release the EFI explicitly from xfs_bmap_finish(). If the EFI
transaction is cancelled, release the EFI in the unlock handler.

Once the EFD is constructed, it is responsible for releasing the EFI
under any circumstances (including whether the EFI item aborts due
to log I/O error). Update the EFD item handlers to release the EFI
if the transaction is cancelled or aborts due to log I/O error.
Finally, update xfs_bmap_finish() to log at least one EFD extent to
the transaction before xfs_free_extent() errors are handled to
ensure the transaction is dirty and EFD item error handling is
triggered.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

1 files changed, 40 insertions, 29 deletions

diff --git a/fs/xfs/xfs_extfree_item.c b/fs/xfs/xfs_extfree_item.c
index 6ff738fe331a..475b9f00ae23 100644
--- a/fs/xfs/xfs_extfree_item.c
+++ b/fs/xfs/xfs_extfree_item.c
@@ -61,9 +61,15 @@ __xfs_efi_release(
 
 	if (atomic_dec_and_test(&efip->efi_refcount)) {
 		spin_lock(&ailp->xa_lock);
-		/* xfs_trans_ail_delete() drops the AIL lock. */
-		xfs_trans_ail_delete(ailp, &efip->efi_item,
-				     SHUTDOWN_LOG_IO_ERROR);
+		/*
+		 * We don't know whether the EFI made it to the AIL. Remove it
+		 * if so. Note that xfs_trans_ail_delete() drops the AIL lock.
+		 */
+		if (efip->efi_item.li_flags & XFS_LI_IN_AIL)
+			xfs_trans_ail_delete(ailp, &efip->efi_item,
+					     SHUTDOWN_LOG_IO_ERROR);
+		else
+			spin_unlock(&ailp->xa_lock);
 		xfs_efi_item_free(efip);
 	}
 }
@@ -128,12 +134,12 @@ xfs_efi_item_pin(
 }
 
 /*
- * While EFIs cannot really be pinned, the unpin operation is the last place at
- * which the EFI is manipulated during a transaction.  If we are being asked to
- * remove the EFI it's because the transaction has been cancelled and by
- * definition that means the EFI cannot be in the AIL so remove it from the
- * transaction and free it.  Otherwise coordinate with xfs_efi_release()
- * to determine who gets to free the EFI.
+ * The unpin operation is the last place an EFI is manipulated in the log. It is
+ * either inserted in the AIL or aborted in the event of a log I/O error. In
+ * either case, the EFI transaction has been successfully committed to make it
+ * this far. Therefore, we expect whoever committed the EFI to either construct
+ * and commit the EFD or drop the EFD's reference in the event of error. Simply
+ * drop the log's EFI reference now that the log is done with it.
  */
 STATIC void
 xfs_efi_item_unpin(
@@ -141,14 +147,6 @@ xfs_efi_item_unpin(
 	int			remove)
 {
 	struct xfs_efi_log_item	*efip = EFI_ITEM(lip);
-
-	if (remove) {
-		ASSERT(!(lip->li_flags & XFS_LI_IN_AIL));
-		if (lip->li_desc)
-			xfs_trans_del_item(lip);
-		xfs_efi_item_free(efip);
-		return;
-	}
 	xfs_efi_release(efip);
 }
 
@@ -167,6 +165,11 @@ xfs_efi_item_push(
 	return XFS_ITEM_PINNED;
 }
 
+/*
+ * The EFI has been either committed or aborted if the transaction has been
+ * cancelled. If the transaction was cancelled, an EFD isn't going to be
+ * constructed and thus we free the EFI here directly.
+ */
 STATIC void
 xfs_efi_item_unlock(
 	struct xfs_log_item	*lip)
@@ -412,20 +415,27 @@ xfs_efd_item_push(
 	return XFS_ITEM_PINNED;
 }
 
+/*
+ * The EFD is either committed or aborted if the transaction is cancelled. If
+ * the transaction is cancelled, drop our reference to the EFI and free the EFD.
+ */
 STATIC void
 xfs_efd_item_unlock(
 	struct xfs_log_item	*lip)
 {
-	if (lip->li_flags & XFS_LI_ABORTED)
-		xfs_efd_item_free(EFD_ITEM(lip));
+	struct xfs_efd_log_item	*efdp = EFD_ITEM(lip);
+
+	if (lip->li_flags & XFS_LI_ABORTED) {
+		xfs_efi_release(efdp->efd_efip);
+		xfs_efd_item_free(efdp);
+	}
 }
 
 /*
- * When the efd item is committed to disk, all we need to do
- * is delete our reference to our partner efi item and then
- * free ourselves.  Since we're freeing ourselves we must
- * return -1 to keep the transaction code from further referencing
- * this item.
+ * When the efd item is committed to disk, all we need to do is delete our
+ * reference to our partner efi item and then free ourselves. Since we're
+ * freeing ourselves we must return -1 to keep the transaction code from further
+ * referencing this item.
  */
 STATIC xfs_lsn_t
 xfs_efd_item_committed(
@@ -435,13 +445,14 @@ xfs_efd_item_committed(
 	struct xfs_efd_log_item	*efdp = EFD_ITEM(lip);
 
 	/*
-	 * If we got a log I/O error, it's always the case that the LR with the
-	 * EFI got unpinned and freed before the EFD got aborted.
+	 * Drop the EFI reference regardless of whether the EFD has been
+	 * aborted. Once the EFD transaction is constructed, it is the sole
+	 * responsibility of the EFD to release the EFI (even if the EFI is
+	 * aborted due to log I/O error).
 	 */
-	if (!(lip->li_flags & XFS_LI_ABORTED))
-		xfs_efi_release(efdp->efd_efip);
-
+	xfs_efi_release(efdp->efd_efip);
 	xfs_efd_item_free(efdp);
+
 	return (xfs_lsn_t)-1;
 }