linux-kernel - Re: [Ocfs2-devel] [PATCH V3] ocfs2: fix dead lock caused by ocfs2_defrag

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Message-ID: <c07c65cb-8492-d866-6a1f-ffa64b293ea0@gmail.com>
Date:   Thu, 1 Nov 2018 16:58:47 +0800
From:   Joseph Qi <jiangqi903@...il.com>
To:     Larry Chen <lchen@...e.com>, mark@...heh.com, jlbec@...lplan.org
Cc:     linux-kernel@...r.kernel.org, ocfs2-devel@....oracle.com
Subject: Re: [Ocfs2-devel] [PATCH V3] ocfs2: fix dead lock caused by
 ocfs2_defrag_extent

Hi Larry,

On 18/11/1 15:14, Larry Chen wrote:
> ocfs2_defrag_extent may fall into deadlock.
> 
> ocfs2_ioctl_move_extents
>     ocfs2_ioctl_move_extents
>       ocfs2_move_extents
>         ocfs2_defrag_extent
>           ocfs2_lock_allocators_move_extents
> 
>             ocfs2_reserve_clusters
>               inode_lock GLOBAL_BITMAP_SYSTEM_INODE
> 
> 	  __ocfs2_flush_truncate_log
>               inode_lock GLOBAL_BITMAP_SYSTEM_INODE
> 
> As backtrace shows above, ocfs2_reserve_clusters() will call inode_lock
> against the global bitmap if local allocator has not sufficient cluters.
> Once global bitmap could meet the demand, ocfs2_reserve_cluster will
> return success with global bitmap locked.
> 
> After ocfs2_reserve_cluster(), if truncate log is full,
> __ocfs2_flush_truncate_log() will definitely fall into deadlock because it
> needs to inode_lock global bitmap, which has already been locked.
> 
> To fix this bug, we could remove from ocfs2_lock_allocators_move_extents()
> the code which intends to lock global allocator, and put the removed code
> after __ocfs2_flush_truncate_log().
> 
> ocfs2_lock_allocators_move_extents() is referred by 2 places, one is here,
> the other does not need the data allocator context, which means this patch
> does not affect the caller so far.
> 
> Change log:
> 1. Correct the function comment.
> 2. Remove unused argument from ocfs2_lock_meta_allocator_move_extents.
> 
> Signed-off-by: Larry Chen <lchen@...e.com>
> ---
>  fs/ocfs2/move_extents.c | 47 ++++++++++++++++++++++++++---------------------
>  1 file changed, 26 insertions(+), 21 deletions(-)
> 
> diff --git a/fs/ocfs2/move_extents.c b/fs/ocfs2/move_extents.c
> index 7eb3b0a6347e..f55f82ca3425 100644
> --- a/fs/ocfs2/move_extents.c
> +++ b/fs/ocfs2/move_extents.c
> @@ -156,18 +156,14 @@ static int __ocfs2_move_extent(handle_t *handle,
>  }
>  
>  /*
> - * lock allocators, and reserving appropriate number of bits for
> - * meta blocks and data clusters.
> - *
> - * in some cases, we don't need to reserve clusters, just let data_ac
> - * be NULL.
> + * lock allocator, and reserve appropriate number of bits for
> + * meta blocks.
>   */
> -static int ocfs2_lock_allocators_move_extents(struct inode *inode,
> +static int ocfs2_lock_meta_allocator_move_extents(struct inode *inode,
>  					struct ocfs2_extent_tree *et,
>  					u32 clusters_to_move,

IMO, here clusters_to_move is only for data_ac, since we change this
function to only handle meta_ac, I'm afraid clusters_to_move related
logic has to been changed correspondingly.

Thanks,
Joseph
>  					u32 extents_to_split,
>  					struct ocfs2_alloc_context **meta_ac,
> -					struct ocfs2_alloc_context **data_ac,
>  					int extra_blocks,
>  					int *credits)
>  {
> @@ -192,13 +188,6 @@ static int ocfs2_lock_allocators_move_extents(struct inode *inode,
>  		goto out;
>  	}
>  
> -	if (data_ac) {
> -		ret = ocfs2_reserve_clusters(osb, clusters_to_move, data_ac);
> -		if (ret) {
> -			mlog_errno(ret);
> -			goto out;
> -		}
> -	}
>  
>  	*credits += ocfs2_calc_extend_credits(osb->sb, et->et_root_el);
>  
> @@ -257,10 +246,10 @@ static int ocfs2_defrag_extent(struct ocfs2_move_extents_context *context,
>  		}
>  	}
>  
> -	ret = ocfs2_lock_allocators_move_extents(inode, &context->et, *len, 1,
> -						 &context->meta_ac,
> -						 &context->data_ac,
> -						 extra_blocks, &credits);
> +	ret = ocfs2_lock_meta_allocator_move_extents(inode, &context->et,
> +						*len, 1,
> +						&context->meta_ac,
> +						extra_blocks, &credits);
>  	if (ret) {
>  		mlog_errno(ret);
>  		goto out;
> @@ -283,6 +272,21 @@ static int ocfs2_defrag_extent(struct ocfs2_move_extents_context *context,
>  		}
>  	}
>  
> +	/*
> +	 * Make sure ocfs2_reserve_cluster is called after
> +	 * __ocfs2_flush_truncate_log, otherwise, dead lock may happen.
> +	 *
> +	 * If ocfs2_reserve_cluster is called
> +	 * before __ocfs2_flush_truncate_log, dead lock on global bitmap
> +	 * may happen.
> +	 *
> +	 */
> +	ret = ocfs2_reserve_clusters(osb, *len, &context->data_ac);
> +	if (ret) {
> +		mlog_errno(ret);
> +		goto out_unlock_mutex;
> +	}
> +
>  	handle = ocfs2_start_trans(osb, credits);
>  	if (IS_ERR(handle)) {
>  		ret = PTR_ERR(handle);
> @@ -600,9 +604,10 @@ static int ocfs2_move_extent(struct ocfs2_move_extents_context *context,
>  		}
>  	}
>  
> -	ret = ocfs2_lock_allocators_move_extents(inode, &context->et, len, 1,
> -						 &context->meta_ac,
> -						 NULL, extra_blocks, &credits);
> +	ret = ocfs2_lock_meta_allocator_move_extents(inode, &context->et,
> +						len, 1,
> +						&context->meta_ac,
> +						extra_blocks, &credits);
>  	if (ret) {
>  		mlog_errno(ret);
>  		goto out;
>