xfs: allow queued AG intents to drain before scrubbing

When a writer thread executes a chain of log intent items, the AG header
buffer locks will cycle during a transaction roll to get from one intent
item to the next in a chain.  Although scrub takes all AG header buffer
locks, this isn't sufficient to guard against scrub checking an AG while
that writer thread is in the middle of finishing a chain because there's
no higher level locking primitive guarding allocation groups.

When there's a collision, cross-referencing between data structures
(e.g. rmapbt and refcountbt) yields false corruption events; if repair
is running, this results in incorrect repairs, which is catastrophic.

Fix this by adding to the perag structure the count of active intents
and make scrub wait until it has both AG header buffer locks and the
intent counter reaches zero.

One quirk of the drain code is that deferred bmap updates also bump and
drop the intent counter.  A fundamental decision made during the design
phase of the reverse mapping feature is that updates to the rmapbt
records are always made by the same code that updates the primary
metadata.  In other words, callers of bmapi functions expect that the
bmapi functions will queue deferred rmap updates.

Some parts of the reflink code queue deferred refcount (CUI) and bmap
(BUI) updates in the same head transaction, but the deferred work
manager completely finishes the CUI before the BUI work is started.  As
a result, the CUI drops the intent count long before the deferred rmap
(RUI) update even has a chance to bump the intent count.  The only way
to keep the intent count elevated between the CUI and RUI is for the BUI
to bump the counter until the RUI has been created.

A second quirk of the intent drain code is that deferred work items must
increment the intent counter as soon as the work item is added to the
transaction.  When a BUI completes and queues an RUI, the RUI must
increment the counter before the BUI decrements it.  The only way to
accomplish this is to require that the counter be bumped as soon as the
deferred work item is created in memory.

In the next patches we'll improve on this facility, but this patch
provides the basic functionality.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>

1 files changed, 96 insertions, 0 deletions

diff --git a/fs/xfs/xfs_drain.c b/fs/xfs/xfs_drain.c
new file mode 100644
index 000000000000..e8fced914f88
--- /dev/null
+++ b/fs/xfs/xfs_drain.c
@@ -0,0 +1,96 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2022 Oracle.  All Rights Reserved.
+ * Author: Darrick J. Wong <djwong@kernel.org>
+ */
+#include "xfs.h"
+#include "xfs_fs.h"
+#include "xfs_shared.h"
+#include "xfs_format.h"
+#include "xfs_trans_resv.h"
+#include "xfs_mount.h"
+#include "xfs_ag.h"
+#include "xfs_trace.h"
+
+void
+xfs_drain_init(
+	struct xfs_drain	*dr)
+{
+	atomic_set(&dr->dr_count, 0);
+	init_waitqueue_head(&dr->dr_waiters);
+}
+
+void
+xfs_drain_free(struct xfs_drain	*dr)
+{
+	ASSERT(atomic_read(&dr->dr_count) == 0);
+}
+
+/* Increase the pending intent count. */
+static inline void xfs_drain_bump(struct xfs_drain *dr)
+{
+	atomic_inc(&dr->dr_count);
+}
+
+/* Decrease the pending intent count, and wake any waiters, if appropriate. */
+static inline void xfs_drain_drop(struct xfs_drain *dr)
+{
+	if (atomic_dec_and_test(&dr->dr_count) &&
+	    wq_has_sleeper(&dr->dr_waiters))
+		wake_up(&dr->dr_waiters);
+}
+
+/* Are there work items pending? */
+static inline bool xfs_drain_busy(struct xfs_drain *dr)
+{
+	return atomic_read(&dr->dr_count) > 0;
+}
+
+/*
+ * Wait for the pending intent count for a drain to hit zero.
+ *
+ * Callers must not hold any locks that would prevent intents from being
+ * finished.
+ */
+static inline int xfs_drain_wait(struct xfs_drain *dr)
+{
+	return wait_event_killable(dr->dr_waiters, !xfs_drain_busy(dr));
+}
+
+/* Add an item to the pending count. */
+void
+xfs_perag_bump_intents(
+	struct xfs_perag	*pag)
+{
+	trace_xfs_perag_bump_intents(pag, __return_address);
+	xfs_drain_bump(&pag->pag_intents);
+}
+
+/* Remove an item from the pending count. */
+void
+xfs_perag_drop_intents(
+	struct xfs_perag	*pag)
+{
+	trace_xfs_perag_drop_intents(pag, __return_address);
+	xfs_drain_drop(&pag->pag_intents);
+}
+
+/*
+ * Wait for the pending intent count for AG metadata to hit zero.
+ * Callers must not hold any AG header buffers.
+ */
+int
+xfs_perag_drain_intents(
+	struct xfs_perag	*pag)
+{
+	trace_xfs_perag_wait_intents(pag, __return_address);
+	return xfs_drain_wait(&pag->pag_intents);
+}
+
+/* Might someone else be processing intents for this AG? */
+bool
+xfs_perag_intents_busy(
+	struct xfs_perag	*pag)
+{
+	return xfs_drain_busy(&pag->pag_intents);
+}