Commit 81317fde authored Jul 24, 2011 by Josef Bacik Committed by Chris Mason Jul 27, 2011

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>

parent a6591715

fs/btrfs/transaction.c

+9 −2

Original line number	Diff line number	Diff line
		@@ -497,11 +497,18 @@ 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);
		WARN_ON(atomic_read(&cur_trans->num_writers) < 1);

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