1 files changed, 22 insertions, 3 deletions

diff --git a/fs/xfs/libxfs/xfs_btree.c b/fs/xfs/libxfs/xfs_btree.c
index 4c16c8c31fcb..c4649cc624e1 100644
--- a/fs/xfs/libxfs/xfs_btree.c
+++ b/fs/xfs/libxfs/xfs_btree.c
@@ -2913,9 +2913,22 @@ xfs_btree_split_worker(
 }
 
 /*
- * BMBT split requests often come in with little stack to work on. Push
+ * BMBT split requests often come in with little stack to work on so we push
  * them off to a worker thread so there is lots of stack to use. For the other
  * btree types, just call directly to avoid the context switch overhead here.
+ *
+ * Care must be taken here - the work queue rescuer thread introduces potential
+ * AGF <> worker queue deadlocks if the BMBT block allocation has to lock new
+ * AGFs to allocate blocks. A task being run by the rescuer could attempt to
+ * lock an AGF that is already locked by a task queued to run by the rescuer,
+ * resulting in an ABBA deadlock as the rescuer cannot run the lock holder to
+ * release it until the current thread it is running gains the lock.
+ *
+ * To avoid this issue, we only ever queue BMBT splits that don't have an AGF
+ * already locked to allocate from. The only place that doesn't hold an AGF
+ * locked is unwritten extent conversion at IO completion, but that has already
+ * been offloaded to a worker thread and hence has no stack consumption issues
+ * we have to worry about.
  */
 STATIC int					/* error */
 xfs_btree_split(
@@ -2929,7 +2942,8 @@ xfs_btree_split(
 	struct xfs_btree_split_args	args;
 	DECLARE_COMPLETION_ONSTACK(done);
 
-	if (cur->bc_btnum != XFS_BTNUM_BMAP)
+	if (cur->bc_btnum != XFS_BTNUM_BMAP ||
+	    cur->bc_tp->t_highest_agno == NULLAGNUMBER)
 		return __xfs_btree_split(cur, level, ptrp, key, curp, stat);
 
 	args.cur = cur;
@@ -4666,7 +4680,12 @@ xfs_btree_space_to_height(
 	const unsigned int	*limits,
 	unsigned long long	leaf_blocks)
 {
-	unsigned long long	node_blocks = limits[1];
+	/*
+	 * The root btree block can have fewer than minrecs pointers in it
+	 * because the tree might not be big enough to require that amount of
+	 * fanout. Hence it has a minimum size of 2 pointers, not limits[1].
+	 */
+	unsigned long long	node_blocks = 2;
 	unsigned long long	blocks_left = leaf_blocks - 1;
 	unsigned int		height = 1;