linux-kernel - [PATCH V2] ocfs2: fix dead lock caused by ocfs2_defrag

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Date:   Sun,  2 Sep 2018 17:14:55 +0800
From:   Larry Chen <lchen@...e.com>
To:     mark@...heh.com, jlbec@...lplan.org
Cc:     linux-kernel@...r.kernel.org, ocfs2-devel@....oracle.com,
        akpm@...ux-foundation.org
Subject: [PATCH V2] ocfs2: fix dead lock caused by ocfs2_defrag_extent

ocfs2_defrag_extent may fall into dead lock.

ocfs2_ioctl_move_extents
  ocfs2_ioctl_move_extents
    ocfs2_move_extents
      ocfs2_defrag_extent
        ocfs2_lock_allocators_move_extents

          ocfs2_reserve_clusters
            inode_lock GLOBAL_BITMAP_SYSTEM_INODE

	  __ocfs2_flush_truncate_log
            inode_lock GLOBAL_BITMAP_SYSTEM_INODE

As back trace shows above, ocfs2_reserve_clusters will call inode_lock
against the global bitmap if local allocator has not sufficient cluters.
Once global bitmap could meet the demand, ocfs2_reserve_cluster will
return success with global bitmap locked.

After ocfs2_reserve_cluster, if truncate log is full,
__ocfs2_flush_truncate_log will definitely fall into dead lock
because it needs to inode_lock global bitmap, which has
already been locked.

To fix this bug, we could remove from ocfs2_lock_allocators_move_extents
the code which intends to lock global allocator, and put the removed
code after __ocfs2_flush_truncate_log

The ocfs2_lock_allocators_move_extents has been refered by 2 places, one
is here, the other does not need the data allocator context, which means
this patch does not affect the caller so far.


Change log:
1. rename ocfs2_lock_allocators_move_extents to
   ocfs2_lock_meta_allocator_move_extents

2. add some comments

Signed-off-by: Larry Chen <lchen@...e.com>
---
 fs/ocfs2/move_extents.c | 40 +++++++++++++++++++++++++---------------
 1 file changed, 25 insertions(+), 15 deletions(-)

diff --git a/fs/ocfs2/move_extents.c b/fs/ocfs2/move_extents.c
index 7eb3b0a6347e..d85dc8a02bd6 100644
--- a/fs/ocfs2/move_extents.c
+++ b/fs/ocfs2/move_extents.c
@@ -162,7 +162,7 @@ static int __ocfs2_move_extent(handle_t *handle,
  * in some cases, we don't need to reserve clusters, just let data_ac
  * be NULL.
  */
-static int ocfs2_lock_allocators_move_extents(struct inode *inode,
+static int ocfs2_lock_meta_allocator_move_extents(struct inode *inode,
 					struct ocfs2_extent_tree *et,
 					u32 clusters_to_move,
 					u32 extents_to_split,
@@ -192,13 +192,6 @@ static int ocfs2_lock_allocators_move_extents(struct inode *inode,
 		goto out;
 	}
 
-	if (data_ac) {
-		ret = ocfs2_reserve_clusters(osb, clusters_to_move, data_ac);
-		if (ret) {
-			mlog_errno(ret);
-			goto out;
-		}
-	}
 
 	*credits += ocfs2_calc_extend_credits(osb->sb, et->et_root_el);
 
@@ -257,10 +250,11 @@ static int ocfs2_defrag_extent(struct ocfs2_move_extents_context *context,
 		}
 	}
 
-	ret = ocfs2_lock_allocators_move_extents(inode, &context->et, *len, 1,
-						 &context->meta_ac,
-						 &context->data_ac,
-						 extra_blocks, &credits);
+	ret = ocfs2_lock_meta_allocator_move_extents(inode, &context->et,
+						*len, 1,
+						&context->meta_ac,
+						&context->data_ac,
+						extra_blocks, &credits);
 	if (ret) {
 		mlog_errno(ret);
 		goto out;
@@ -283,6 +277,21 @@ static int ocfs2_defrag_extent(struct ocfs2_move_extents_context *context,
 		}
 	}
 
+	/*
+	 * Make sure ocfs2_reserve_cluster is called after
+	 * __ocfs2_flush_truncate_log, otherwise, dead lock may happen.
+	 *
+	 * If ocfs2_reserve_cluster is called
+	 * before __ocfs2_flush_truncate_log, dead lock on global bitmap
+	 * may happen.
+	 *
+	 */
+	ret = ocfs2_reserve_clusters(osb, *len, &context->data_ac);
+	if (ret) {
+		mlog_errno(ret);
+		goto out_unlock_mutex;
+	}
+
 	handle = ocfs2_start_trans(osb, credits);
 	if (IS_ERR(handle)) {
 		ret = PTR_ERR(handle);
@@ -600,9 +609,10 @@ static int ocfs2_move_extent(struct ocfs2_move_extents_context *context,
 		}
 	}
 
-	ret = ocfs2_lock_allocators_move_extents(inode, &context->et, len, 1,
-						 &context->meta_ac,
-						 NULL, extra_blocks, &credits);
+	ret = ocfs2_lock_meta_allocator_move_extents(inode, &context->et,
+						len, 1,
+						&context->meta_ac,
+						NULL, extra_blocks, &credits);
 	if (ret) {
 		mlog_errno(ret);
 		goto out;
-- 
2.13.7