Btrfs: set trans to null in reserve_metadata_bytes if we commit the transaction

btrfs_commit_transaction will free our trans, but because we pass trans to
shrink_delalloc we could possibly have a use after free situation.  So instead
if we commit the transaction, set trans to null and set committed to true so we
don't keep trying to commit a transaction.  This fixes a panic I could reproduce
at will.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>

1 files changed, 6 insertions, 3 deletions

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 180a50146ddf..e2dfd4ab3b9b 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -3157,6 +3157,7 @@ static int reserve_metadata_bytes(struct btrfs_trans_handle *trans,
         int retries = 0;
         int ret = 0;
         bool reserved = false;
+        bool committed = false;
 
 again:
         ret = -ENOSPC;
@@ -3249,17 +3250,19 @@ again:
                 goto out;
 
         ret = -EAGAIN;
-        if (trans)
+        if (trans || committed)
                 goto out;
 
-
         ret = -ENOSPC;
         trans = btrfs_join_transaction(root, 1);
         if (IS_ERR(trans))
                 goto out;
         ret = btrfs_commit_transaction(trans, root);
-        if (!ret)
+        if (!ret) {
+                trans = NULL;
+                committed = true;
                 goto again;
+        }
 
 out:
         if (reserved) {