Btrfs: fix deadlock when throttling transactions

Hit this nice little deadlock.  What happens is this

__btrfs_end_transaction with throttle set, --use_count so it equals 0
  btrfs_commit_transaction
    <somebody else actually manages to start the commit>
    btrfs_end_transaction --use_count so now its -1 <== BAD
      we just return and wait on the transaction

This is bad because we just return after our use_count is -1 and don't let go
of our num_writer count on the transaction, so the guy committing the
transaction just sits there forever.  Fix this by inc'ing our use_count if we're
going to call commit_transaction so that if we call btrfs_end_transaction it's
valid.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 9 insertions, 2 deletions

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 654755b18951..eb55863bb4ae 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -497,10 +497,17 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
         }
 
         if (lock && cur_trans->blocked && !cur_trans->in_commit) {
-                if (throttle)
+                if (throttle) {
+                        /*
+                         * We may race with somebody else here so end up having
+                         * to call end_transaction on ourselves again, so inc
+                         * our use_count.
+                         */
+                        trans->use_count++;
                         return btrfs_commit_transaction(trans, root);
-                else
+                } else {
                         wake_up_process(info->transaction_kthread);
+                }
         }
 
         WARN_ON(cur_trans != info->running_transaction);