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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>
This commit is contained in:
Josef Bacik 2011-07-24 15:45:34 -04:00 committed by Chris Mason
parent a65917156e
commit 81317fdedd
1 changed files with 9 additions and 2 deletions
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11
fs/btrfs/transaction.c
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@ -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);