From mboxrd@z Thu Jan  1 00:00:00 1970
From: liubo <liubo2009@cn.fujitsu.com>
Subject: Re: [PATCH] Btrfs: fix deadlock when throttling transactions
Date: Fri, 15 Jul 2011 10:56:42 +0800
Message-ID: <4E1FAC6A.3050501@cn.fujitsu.com>
References: <1310664370-20091-1-git-send-email-josef@redhat.com>
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Cc: linux-btrfs@vger.kernel.org
To: Josef Bacik <josef@redhat.com>
Return-path: <linux-btrfs-owner@vger.kernel.org>
In-Reply-To: <1310664370-20091-1-git-send-email-josef@redhat.com>
List-ID: <linux-btrfs.vger.kernel.org>

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
>     <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>
> ---
>  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).

thanks,
liubo

>  		ret = wait_for_commit(root, cur_trans);
>  		BUG_ON(ret);