[PATCH 2/4] xfs: avoid buffer deadlocks when walking fs inodes

["Darrick J. Wong" <djwong@kernel.org>]

From: Darrick J. Wong <djwong@kernel.org>

When we're servicing an INUMBERS or BULKSTAT request or running
quotacheck, grab an empty transaction so that we can use its inherent
recursive buffer locking abilities to detect inode btree cycles without
hitting ABBA buffer deadlocks.

Found by fuzzing an inode btree pointer to introduce a cycle into the
tree (xfs/365).

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/xfs/xfs_itable.c |   40 ++++++++++++++++++++++++++++++++++++----
 fs/xfs/xfs_iwalk.c  |   32 +++++++++++++++++++++++++++-----
 2 files changed, 63 insertions(+), 9 deletions(-)


diff --git a/fs/xfs/xfs_itable.c b/fs/xfs/xfs_itable.c
index ca310a125d1e..0cc19135e827 100644
--- a/fs/xfs/xfs_itable.c
+++ b/fs/xfs/xfs_itable.c
@@ -19,6 +19,7 @@
 #include "xfs_error.h"
 #include "xfs_icache.h"
 #include "xfs_health.h"
+#include "xfs_trans.h"
 
 /*
  * Bulk Stat
@@ -166,6 +167,7 @@ xfs_bulkstat_one(
 		.formatter	= formatter,
 		.breq		= breq,
 	};
+	struct xfs_trans	*tp;
 	int			error;
 
 	ASSERT(breq->icount == 1);
@@ -175,9 +177,18 @@ xfs_bulkstat_one(
 	if (!bc.buf)
 		return -ENOMEM;
 
+	/*
+	 * Grab an empty transaction so that we can use its recursive buffer
+	 * locking abilities to detect cycles in the inobt without deadlocking.
+	 */
+	error = xfs_trans_alloc_empty(breq->mp, &tp);
+	if (error)
+		goto out;
+
 	error = xfs_bulkstat_one_int(breq->mp, breq->mnt_userns, NULL,
 				     breq->startino, &bc);
-
+	xfs_trans_cancel(tp);
+out:
 	kmem_free(bc.buf);
 
 	/*
@@ -241,6 +252,7 @@ xfs_bulkstat(
 		.formatter	= formatter,
 		.breq		= breq,
 	};
+	struct xfs_trans	*tp;
 	int			error;
 
 	if (breq->mnt_userns != &init_user_ns) {
@@ -256,9 +268,18 @@ xfs_bulkstat(
 	if (!bc.buf)
 		return -ENOMEM;
 
-	error = xfs_iwalk(breq->mp, NULL, breq->startino, breq->flags,
+	/*
+	 * Grab an empty transaction so that we can use its recursive buffer
+	 * locking abilities to detect cycles in the inobt without deadlocking.
+	 */
+	error = xfs_trans_alloc_empty(breq->mp, &tp);
+	if (error)
+		goto out;
+
+	error = xfs_iwalk(breq->mp, tp, breq->startino, breq->flags,
 			xfs_bulkstat_iwalk, breq->icount, &bc);
-
+	xfs_trans_cancel(tp);
+out:
 	kmem_free(bc.buf);
 
 	/*
@@ -371,13 +392,24 @@ xfs_inumbers(
 		.formatter	= formatter,
 		.breq		= breq,
 	};
+	struct xfs_trans	*tp;
 	int			error = 0;
 
 	if (xfs_bulkstat_already_done(breq->mp, breq->startino))
 		return 0;
 
-	error = xfs_inobt_walk(breq->mp, NULL, breq->startino, breq->flags,
+	/*
+	 * Grab an empty transaction so that we can use its recursive buffer
+	 * locking abilities to detect cycles in the inobt without deadlocking.
+	 */
+	error = xfs_trans_alloc_empty(breq->mp, &tp);
+	if (error)
+		goto out;
+
+	error = xfs_inobt_walk(breq->mp, tp, breq->startino, breq->flags,
 			xfs_inumbers_walk, breq->icount, &ic);
+	xfs_trans_cancel(tp);
+out:
 
 	/*
 	 * We found some inode groups, so clear the error status and return
diff --git a/fs/xfs/xfs_iwalk.c b/fs/xfs/xfs_iwalk.c
index c4a340f1f1e1..e1e889f3647f 100644
--- a/fs/xfs/xfs_iwalk.c
+++ b/fs/xfs/xfs_iwalk.c
@@ -81,6 +81,9 @@ struct xfs_iwalk_ag {
 
 	/* Skip empty inobt records? */
 	unsigned int			skip_empty:1;
+
+	/* Drop the (hopefully empty) transaction when calling iwalk_fn. */
+	unsigned int			drop_trans:1;
 };
 
 /*
@@ -351,7 +354,6 @@ xfs_iwalk_run_callbacks(
 	int				*has_more)
 {
 	struct xfs_mount		*mp = iwag->mp;
-	struct xfs_trans		*tp = iwag->tp;
 	struct xfs_inobt_rec_incore	*irec;
 	xfs_agino_t			next_agino;
 	int				error;
@@ -361,10 +363,15 @@ xfs_iwalk_run_callbacks(
 	ASSERT(iwag->nr_recs > 0);
 
 	/* Delete cursor but remember the last record we cached... */
-	xfs_iwalk_del_inobt(tp, curpp, agi_bpp, 0);
+	xfs_iwalk_del_inobt(iwag->tp, curpp, agi_bpp, 0);
 	irec = &iwag->recs[iwag->nr_recs - 1];
 	ASSERT(next_agino >= irec->ir_startino + XFS_INODES_PER_CHUNK);
 
+	if (iwag->drop_trans) {
+		xfs_trans_cancel(iwag->tp);
+		iwag->tp = NULL;
+	}
+
 	error = xfs_iwalk_ag_recs(iwag);
 	if (error)
 		return error;
@@ -375,8 +382,14 @@ xfs_iwalk_run_callbacks(
 	if (!has_more)
 		return 0;
 
+	if (iwag->drop_trans) {
+		error = xfs_trans_alloc_empty(mp, &iwag->tp);
+		if (error)
+			return error;
+	}
+
 	/* ...and recreate the cursor just past where we left off. */
-	error = xfs_inobt_cur(mp, tp, agno, XFS_BTNUM_INO, curpp, agi_bpp);
+	error = xfs_inobt_cur(mp, iwag->tp, agno, XFS_BTNUM_INO, curpp, agi_bpp);
 	if (error)
 		return error;
 
@@ -389,7 +402,6 @@ xfs_iwalk_ag(
 	struct xfs_iwalk_ag		*iwag)
 {
 	struct xfs_mount		*mp = iwag->mp;
-	struct xfs_trans		*tp = iwag->tp;
 	struct xfs_buf			*agi_bp = NULL;
 	struct xfs_btree_cur		*cur = NULL;
 	xfs_agnumber_t			agno;
@@ -469,7 +481,7 @@ xfs_iwalk_ag(
 	error = xfs_iwalk_run_callbacks(iwag, agno, &cur, &agi_bp, &has_more);
 
 out:
-	xfs_iwalk_del_inobt(tp, &cur, &agi_bp, error);
+	xfs_iwalk_del_inobt(iwag->tp, &cur, &agi_bp, error);
 	return error;
 }
 
@@ -594,8 +606,18 @@ xfs_iwalk_ag_work(
 	error = xfs_iwalk_alloc(iwag);
 	if (error)
 		goto out;
+	/*
+	 * Grab an empty transaction so that we can use its recursive buffer
+	 * locking abilities to detect cycles in the inobt without deadlocking.
+	 */
+	error = xfs_trans_alloc_empty(mp, &iwag->tp);
+	if (error)
+		goto out;
+	iwag->drop_trans = 1;
 
 	error = xfs_iwalk_ag(iwag);
+	if (iwag->tp)
+		xfs_trans_cancel(iwag->tp);
 	xfs_iwalk_free(iwag);
 out:
 	kmem_free(iwag);