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authorBrian Foster <bfoster@redhat.com>2018-09-29 13:45:26 +1000
committerDave Chinner <david@fromorbit.com>2018-09-29 13:45:26 +1000
commit95808459b110f16b50f03a70ecfa72bb14bd8a96 (patch)
tree10f08200523405f5b534af39552d3061a348e118 /fs/xfs/xfs_buf_item.c
parent23420d05e67d23728e116321c4afe084ebfa6427 (diff)
xfs: refactor xfs_buf_log_item reference count handling
The xfs_buf_log_item structure has a reference counter with slightly tricky semantics. In the common case, a buffer is logged and committed in a transaction, committed to the on-disk log (added to the AIL) and then finally written back and removed from the AIL. The bli refcount covers two potentially overlapping timeframes: 1. the bli is held in an active transaction 2. the bli is pinned by the log The caveat to this approach is that the reference counter does not purely dictate the lifetime of the bli. IOW, when a dirty buffer is physically logged and unpinned, the bli refcount may go to zero as the log item is inserted into the AIL. Only once the buffer is written back can the bli finally be freed. The above semantics means that it is not enough for the various refcount decrementing contexts to release the bli on decrement to zero. xfs_trans_brelse(), transaction commit (->iop_unlock()) and unpin (->iop_unpin()) must all drop the associated reference and make additional checks to determine if the current context is responsible for freeing the item. For example, if a transaction holds but does not dirty a particular bli, the commit may drop the refcount to zero. If the bli itself is clean, it is also not AIL resident and must be freed at this time. The same is true for xfs_trans_brelse(). If the transaction dirties a bli and then aborts or an unpin results in an abort due to a log I/O error, the last reference count holder is expected to explicitly remove the item from the AIL and release it (since an abort means filesystem shutdown and metadata writeback will never occur). This leads to fairly complex checks being replicated in a few different places. Since ->iop_unlock() and xfs_trans_brelse() are nearly identical, refactor the logic into a common helper that implements and documents the semantics in one place. This patch does not change behavior. Signed-off-by: Brian Foster <bfoster@redhat.com> Reviewed-by: Dave Chinner <dchinner@redhat.com> Signed-off-by: Dave Chinner <david@fromorbit.com>
Diffstat (limited to 'fs/xfs/xfs_buf_item.c')
-rw-r--r--fs/xfs/xfs_buf_item.c90
1 files changed, 53 insertions, 37 deletions
diff --git a/fs/xfs/xfs_buf_item.c b/fs/xfs/xfs_buf_item.c
index 42fce70b474d..12d8455bfbb2 100644
--- a/fs/xfs/xfs_buf_item.c
+++ b/fs/xfs/xfs_buf_item.c
@@ -532,6 +532,49 @@ xfs_buf_item_push(
}
/*
+ * Drop the buffer log item refcount and take appropriate action. This helper
+ * determines whether the bli must be freed or not, since a decrement to zero
+ * does not necessarily mean the bli is unused.
+ *
+ * Return true if the bli is freed, false otherwise.
+ */
+bool
+xfs_buf_item_put(
+ struct xfs_buf_log_item *bip)
+{
+ struct xfs_log_item *lip = &bip->bli_item;
+ bool aborted;
+ bool dirty;
+
+ /* drop the bli ref and return if it wasn't the last one */
+ if (!atomic_dec_and_test(&bip->bli_refcount))
+ return false;
+
+ /*
+ * We dropped the last ref and must free the item if clean or aborted.
+ * If the bli is dirty and non-aborted, the buffer was clean in the
+ * transaction but still awaiting writeback from previous changes. In
+ * that case, the bli is freed on buffer writeback completion.
+ */
+ aborted = test_bit(XFS_LI_ABORTED, &lip->li_flags) ||
+ XFS_FORCED_SHUTDOWN(lip->li_mountp);
+ dirty = bip->bli_flags & XFS_BLI_DIRTY;
+ if (dirty && !aborted)
+ return false;
+
+ /*
+ * The bli is aborted or clean. An aborted item may be in the AIL
+ * regardless of dirty state. For example, consider an aborted
+ * transaction that invalidated a dirty bli and cleared the dirty
+ * state.
+ */
+ if (aborted)
+ xfs_trans_ail_remove(lip, SHUTDOWN_LOG_IO_ERROR);
+ xfs_buf_item_relse(bip->bli_buf);
+ return true;
+}
+
+/*
* Release the buffer associated with the buf log item. If there is no dirty
* logged data associated with the buffer recorded in the buf log item, then
* free the buf log item and remove the reference to it in the buffer.
@@ -556,13 +599,12 @@ xfs_buf_item_unlock(
{
struct xfs_buf_log_item *bip = BUF_ITEM(lip);
struct xfs_buf *bp = bip->bli_buf;
- bool freed;
- bool aborted;
+ bool released;
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;
+ bool dirty = bip->bli_flags & XFS_BLI_DIRTY;
#endif
trace_xfs_buf_item_unlock(bip);
@@ -575,8 +617,6 @@ xfs_buf_item_unlock(
(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);
-
/*
* Clear the buffer's association with this transaction and
* per-transaction state from the bli, which has been copied above.
@@ -585,40 +625,16 @@ xfs_buf_item_unlock(
bip->bli_flags &= ~(XFS_BLI_LOGGED | XFS_BLI_HOLD | XFS_BLI_ORDERED);
/*
- * Drop the transaction's bli reference and deal with the item if we had
- * the last one. We must free the item if clean or aborted since it
- * wasn't pinned by the log and this is the last chance to do so. If the
- * bli is freed and dirty (but non-aborted), the buffer was not dirty in
- * this transaction but modified by a previous one and still awaiting
- * writeback. In that case, the bli is freed on buffer writeback
- * completion.
+ * Unref the item and unlock the buffer unless held or stale. Stale
+ * buffers remain locked until final unpin unless the bli is freed by
+ * the unref call. The latter implies shutdown because buffer
+ * invalidation dirties the bli and transaction.
*/
- freed = atomic_dec_and_test(&bip->bli_refcount);
- if (freed) {
- ASSERT(!aborted || XFS_FORCED_SHUTDOWN(lip->li_mountp));
- /*
- * An aborted item may be in the AIL regardless of dirty state.
- * For example, consider an aborted transaction that invalidated
- * a dirty bli and cleared the dirty state.
- */
- if (aborted)
- xfs_trans_ail_remove(lip, SHUTDOWN_LOG_IO_ERROR);
- if (aborted || !dirty)
- xfs_buf_item_relse(bp);
- } else if (stale) {
- /*
- * Stale buffers remain locked until final unpin unless the bli
- * was freed in the branch above. A freed stale bli implies an
- * abort because buffer invalidation dirties the bli and
- * transaction.
- */
- ASSERT(!freed);
+ released = xfs_buf_item_put(bip);
+ if (hold || (stale && !released))
return;
- }
- ASSERT(!stale || (aborted && freed));
-
- if (!hold)
- xfs_buf_relse(bp);
+ ASSERT(!stale || test_bit(XFS_LI_ABORTED, &lip->li_flags));
+ xfs_buf_relse(bp);
}
/*