[PATCH] btrfs: Take trans lock before access running trans in check_delayed_ref

[ethanwu <ethanwu@xxxxxxxxxxxx>]

In preivous patch:
Btrfs: kill trans in run_delalloc_nocow and btrfs_cross_ref_exist
We avoid starting btrfs transaction and get transaction from
fs_info->running_transaction directly.

When accessing running_transaction in check_delayed_ref, there's a
chance that current transaction will be freed by commit transaction
after the NULL pointer check of running_transaction is passed.

After looking all the other places using fs_info->running_transaction,
they are either
1. protected by trans_lock or
2. holding the transaction, so commit transaction must wait before
   it could finish committing and free the transaction structure

Fix this by using trans_lock and increasing the use_count.

Fixes: e4c3b2dcd144 ("Btrfs: kill trans in run_delalloc_nocow and btrfs_cross_ref_exist")
Signed-off-by: ethanwu <ethanwu@xxxxxxxxxxxx>
Reviewed-by: Liu Bo <bo.liu@xxxxxxxxxxxxxxxxx>
---
V2: Use refcount_inc rather than aomitc_inc to increase use_count
V3: Add Fixes tag and refine commit log

 fs/btrfs/extent-tree.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index c1618ab..7c302ed 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -3149,7 +3149,11 @@ static noinline int check_delayed_ref(struct btrfs_root *root,
 	struct rb_node *node;
 	int ret = 0;
 
+	spin_lock(&root->fs_info->trans_lock);
 	cur_trans = root->fs_info->running_transaction;
+	if (cur_trans)
+		refcount_inc(&cur_trans->use_count);
+	spin_unlock(&root->fs_info->trans_lock);
 	if (!cur_trans)
 		return 0;
 
@@ -3158,6 +3162,7 @@ static noinline int check_delayed_ref(struct btrfs_root *root,
 	head = btrfs_find_delayed_ref_head(delayed_refs, bytenr);
 	if (!head) {
 		spin_unlock(&delayed_refs->lock);
+		btrfs_put_transaction(cur_trans);
 		return 0;
 	}
 
@@ -3174,6 +3179,7 @@ static noinline int check_delayed_ref(struct btrfs_root *root,
 		mutex_lock(&head->mutex);
 		mutex_unlock(&head->mutex);
 		btrfs_put_delayed_ref_head(head);
+		btrfs_put_transaction(cur_trans);
 		return -EAGAIN;
 	}
 	spin_unlock(&delayed_refs->lock);
@@ -3206,6 +3212,7 @@ static noinline int check_delayed_ref(struct btrfs_root *root,
 	}
 	spin_unlock(&head->lock);
 	mutex_unlock(&head->mutex);
+	btrfs_put_transaction(cur_trans);
 	return ret;
 }
 
-- 
1.9.1

--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html