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

[ethanwu]

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 <etha...@synology.com>
Reviewed-by: Liu Bo <bo....@linux.alibaba.com>
---
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

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