From mboxrd@z Thu Jan 1 00:00:00 1970 From: Josef Bacik Subject: Re: [PATCH] Btrfs: fix deadlock when throttling transactions Date: Fri, 15 Jul 2011 09:54:55 -0400 Message-ID: <4E2046AF.3030405@redhat.com> References: <1310664370-20091-1-git-send-email-josef@redhat.com> <4E1FAC6A.3050501@cn.fujitsu.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Cc: linux-btrfs@vger.kernel.org To: liubo Return-path: In-Reply-To: <4E1FAC6A.3050501@cn.fujitsu.com> List-ID: On 07/14/2011 10:56 PM, liubo wrote: > On 07/15/2011 01:26 AM, Josef Bacik wrote: >> Hit this nice little deadlock. What happens is this >> >> __btrfs_end_transaction with throttle set, --use_count so it equals 0 >> btrfs_commit_transaction >> >> 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 >> --- >> fs/btrfs/transaction.c | 13 ++++++++++--- >> 1 files changed, 10 insertions(+), 3 deletions(-) >> >> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c >> index 654755b..00b81fb5 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); >> @@ -1225,7 +1232,7 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans, >> if (cur_trans->in_commit) { >> spin_unlock(&cur_trans->commit_lock); >> atomic_inc(&cur_trans->use_count); >> - btrfs_end_transaction(trans, root); >> + __btrfs_end_transaction(trans, root, 0, 1); >> > > Looks good. > > BTW, btrfs_end_transaction(trans, root) is just __btrfs_end_transaction(trans, root, 0, 1). > Oops you're right, I saw the 1 for lock and thought it was for throttle. Thanks, Josef