Re: [PATCH 3/3] ext4: Fix deadlock when writing in ENOSPC conditions

[Zheng Liu <gnehzuil.liu@gmail.com>]

On Tue, Dec 10, 2013 at 11:14:00AM +0100, Jan Kara wrote:
> Akira-san has been reporting rare deadlocks of his machine when running
> xfstests test 269 on ext4 filesystem. The problem turned out to be in
> ext4_da_reserve_metadata() and ext4_da_reserve_space() which called
> ext4_should_retry_alloc() while holding i_data_sem. Since
> ext4_should_retry_alloc() can force a transaction commit, this is a
> lock ordering violation and leads to deadlocks.
> 
> Fix the problem by just removing the retry loops. These functions should
> just report ENOSPC to the caller (e.g. ext4_da_write_begin()) and that
> function must take care of retrying after dropping all necessary locks.
> 
> Reported-and-tested-by: Akira Fujita <a-fujita@rs.jp.nec.com>
> Signed-off-by: Jan Kara <jack@suse.cz>

Thanks for fixing this.  The patch looks good to me.  You can add:
Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>

BTW, I have met a deadlock which is caused by ext4_da_reserve_space()
in our product system.  The calltrace information looks like this.  So
I want to make sure it is the root cause.  But I couldn't reproduce the
problem with running xfstest #269.  Could you please tell me how to
reproduce the deadlock?

FWIW, I think we should backport this patch to stable kernel.

Thanks,
                                                - Zheng

> ---
>  fs/ext4/inode.c | 12 ------------
>  1 file changed, 12 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 075763474118..61d49ff22c81 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -1206,7 +1206,6 @@ static int ext4_journalled_write_end(struct file *file,
>   */
>  static int ext4_da_reserve_metadata(struct inode *inode, ext4_lblk_t lblock)
>  {
> -	int retries = 0;
>  	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
>  	struct ext4_inode_info *ei = EXT4_I(inode);
>  	unsigned int md_needed;
> @@ -1218,7 +1217,6 @@ static int ext4_da_reserve_metadata(struct inode *inode, ext4_lblk_t lblock)
>  	 * in order to allocate nrblocks
>  	 * worse case is one extent per block
>  	 */
> -repeat:
>  	spin_lock(&ei->i_block_reservation_lock);
>  	/*
>  	 * ext4_calc_metadata_amount() has side effects, which we have
> @@ -1238,10 +1236,6 @@ repeat:
>  		ei->i_da_metadata_calc_len = save_len;
>  		ei->i_da_metadata_calc_last_lblock = save_last_lblock;
>  		spin_unlock(&ei->i_block_reservation_lock);
> -		if (ext4_should_retry_alloc(inode->i_sb, &retries)) {
> -			cond_resched();
> -			goto repeat;
> -		}
>  		return -ENOSPC;
>  	}
>  	ei->i_reserved_meta_blocks += md_needed;
> @@ -1255,7 +1249,6 @@ repeat:
>   */
>  static int ext4_da_reserve_space(struct inode *inode, ext4_lblk_t lblock)
>  {
> -	int retries = 0;
>  	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
>  	struct ext4_inode_info *ei = EXT4_I(inode);
>  	unsigned int md_needed;
> @@ -1277,7 +1270,6 @@ static int ext4_da_reserve_space(struct inode *inode, ext4_lblk_t lblock)
>  	 * in order to allocate nrblocks
>  	 * worse case is one extent per block
>  	 */
> -repeat:
>  	spin_lock(&ei->i_block_reservation_lock);
>  	/*
>  	 * ext4_calc_metadata_amount() has side effects, which we have
> @@ -1297,10 +1289,6 @@ repeat:
>  		ei->i_da_metadata_calc_len = save_len;
>  		ei->i_da_metadata_calc_last_lblock = save_last_lblock;
>  		spin_unlock(&ei->i_block_reservation_lock);
> -		if (ext4_should_retry_alloc(inode->i_sb, &retries)) {
> -			cond_resched();
> -			goto repeat;
> -		}
>  		dquot_release_reservation_block(inode, EXT4_C2B(sbi, 1));
>  		return -ENOSPC;
>  	}
> -- 
> 1.8.1.4
> 
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