Re: [PATCH V3] f2fs: Avoid double lock for cp_rwsem during checkpoint

[Chao Yu]

On 2020/4/30 1:00, Sayali Lokhande wrote:
> There could be a scenario where f2fs_sync_node_pages gets
> called during checkpoint, which in turn tries to flush
> inline data and calls iput(). This results in deadlock as
> iput() tries to hold cp_rwsem, which is already held at the
> beginning by checkpoint->block_operations().
> 
> Call stack :
> 
> Thread A		Thread B
> f2fs_write_checkpoint()
> - block_operations(sbi)
>  - f2fs_lock_all(sbi);
>   - down_write(&sbi->cp_rwsem);
> 
>                         - open()
>                          - igrab()
>                         - write() write inline data
>                         - unlink()
> - f2fs_sync_node_pages()
>  - if (is_inline_node(page))
>   - flush_inline_data()
>    - ilookup()
>      page = f2fs_pagecache_get_page()
>      if (!page)
>       goto iput_out;
>      iput_out:
> 			-close()
> 			-iput()
>        iput(inode);
>        - f2fs_evict_inode()
>         - f2fs_truncate_blocks()
>          - f2fs_lock_op()
>            - down_read(&sbi->cp_rwsem);
> 
> Fixes: 399368372ed9 ("f2fs: introduce a new global lock scheme")

IMO, it should be

2049d4fcb057 ("f2fs: avoid multiple node page writes due to inline_data")

It brings iput() to checkpoint process for the first time.

Thanks,

> Signed-off-by: Sayali Lokhande <sayalil@xxxxxxxxxxxxxx>
> ---
>  fs/f2fs/checkpoint.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
> index 5ba649e..97b6378 100644
> --- a/fs/f2fs/checkpoint.c
> +++ b/fs/f2fs/checkpoint.c
> @@ -1219,21 +1219,19 @@ static int block_operations(struct f2fs_sb_info *sbi)
>  		goto retry_flush_quotas;
>  	}
>  
> -retry_flush_nodes:
>  	down_write(&sbi->node_write);
>  
>  	if (get_pages(sbi, F2FS_DIRTY_NODES)) {
>  		up_write(&sbi->node_write);
> +		up_write(&sbi->node_change);
> +		f2fs_unlock_all(sbi);
>  		atomic_inc(&sbi->wb_sync_req[NODE]);
>  		err = f2fs_sync_node_pages(sbi, &wbc, false, FS_CP_NODE_IO);
>  		atomic_dec(&sbi->wb_sync_req[NODE]);
> -		if (err) {
> -			up_write(&sbi->node_change);
> -			f2fs_unlock_all(sbi);
> +		if (err)
>  			goto out;
> -		}
>  		cond_resched();
> -		goto retry_flush_nodes;
> +		goto retry_flush_quotas;
>  	}
>  
>  	/*
>