[PATCH] xfs: fix the ABBA deadlock around agf and inode

["Wang Jianchao (Kuaishou)" <jianchao.wan9@gmail.com>]

Recently, we encounter a deadlock case where there are many tasks
hung on agi and agf xfs_buf. It end up with a deadlock between
agf and inode xfs_buf as following,

xfs_ifree()             xfs_bmap_btalloc()
xfs_iunlink_remove()    xfs_alloc_vextent()
  hold inode bp           hold agf
xfs_difree()            xfs_trans_log_inode()
  require agf             require inode bp

The task requires inode bp with agf held block other tasks which
want to require agf with agi held. Then the whole filesystem
looks like a agi/agf deadlock. To fix this issue, get agf in
xfs_iunlink_remove() after it get agi and before get indoe bp.

Signed-off-by: Wang Jianchao (Kuaishou) <jianchao.wan9@gmail.com>
---
 fs/xfs/xfs_inode.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
index 9de6205fe134..c0a29fd00b0e 100644
--- a/fs/xfs/xfs_inode.c
+++ b/fs/xfs/xfs_inode.c
@@ -36,6 +36,7 @@
 #include "xfs_reflink.h"
 #include "xfs_ag.h"
 #include "xfs_log_priv.h"
+#include "xfs_alloc.h"
 
 struct kmem_cache *xfs_inode_cache;
 
@@ -2337,6 +2338,7 @@ xfs_iunlink_remove(
 {
 	struct xfs_mount	*mp = tp->t_mountp;
 	struct xfs_agi		*agi;
+	struct xfs_buf		*agfbp;
 	struct xfs_buf		*agibp;
 	struct xfs_buf		*last_ibp;
 	struct xfs_dinode	*last_dip = NULL;
@@ -2352,6 +2354,14 @@ xfs_iunlink_remove(
 	error = xfs_read_agi(mp, tp, pag->pag_agno, &agibp);
 	if (error)
 		return error;
+
+	/*
+	 * Get the agf buffer first to ensure the lock ordering against inode bp
+	 */
+	error = xfs_read_agf(mp, tp, pag->pag_agno, 0, &agfbp);
+	if (error)
+		return error;
+
 	agi = agibp->b_addr;
 
 	/*
-- 
2.17.1