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, 22 insertions, 3 deletions

diff --git a/fs/xfs/xfs_extfree_item.h b/fs/xfs/xfs_extfree_item.h
index 399562eaf4f5..8fa8651705e1 100644
--- a/fs/xfs/xfs_extfree_item.h
+++ b/fs/xfs/xfs_extfree_item.h
@@ -39,9 +39,28 @@ struct kmem_zone;
  * "extent free done" log item described below.
  *
  * The EFI is reference counted so that it is not freed prior to both the EFI
- * and EFD being committed and unpinned. This ensures that when the last
- * reference goes away the EFI will always be in the AIL as it has been
- * unpinned, regardless of whether the EFD is processed before or after the EFI.
+ * and EFD being committed and unpinned. This ensures the EFI is inserted into
+ * the AIL even in the event of out of order EFI/EFD processing. In other words,
+ * an EFI is born with two references:
+ *
+ * 	1.) an EFI held reference to track EFI AIL insertion
+ * 	2.) an EFD held reference to track EFD commit
+ *
+ * On allocation, both references are the responsibility of the caller. Once the
+ * EFI is added to and dirtied in a transaction, ownership of reference one
+ * transfers to the transaction. The reference is dropped once the EFI is
+ * inserted to the AIL or in the event of failure along the way (e.g., commit
+ * failure, log I/O error, etc.). Note that the caller remains responsible for
+ * the EFD reference under all circumstances to this point. The caller has no
+ * means to detect failure once the transaction is committed, however.
+ * Therefore, an EFD is required after this point, even in the event of
+ * unrelated failure.
+ *
+ * Once an EFD is allocated and dirtied in a transaction, reference two
+ * transfers to the transaction. The EFD reference is dropped once it reaches
+ * the unpin handler. Similar to the EFI, the reference also drops in the event
+ * of commit failure or log I/O errors. Note that the EFD is not inserted in the
+ * AIL, so at this point both the EFI and EFD are freed.
  */
 typedef struct xfs_efi_log_item {
 	xfs_log_item_t		efi_item;