linux-kernel - Re: [PATCH v2] f2fs: fix to avoid racing in between read and OPU dio write

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Date: Wed, 15 May 2024 02:32:19 -0600
From: Wu Bo <bo.wu@...o.com>
To: chao@...nel.org
Cc: jaegeuk@...nel.org,
	linux-f2fs-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] f2fs: fix to avoid racing in between read and OPU dio write

On Fri, May 10, 2024 at 10:39:06AM +0800, Chao Yu wrote:
> If lfs mode is on, buffered read may race w/ OPU dio write as below,
> it may cause buffered read hits unwritten data unexpectly, and for
> dio read, the race condition exists as well.
> 
> Thread A			Thread B
> - f2fs_file_write_iter
>  - f2fs_dio_write_iter
>   - __iomap_dio_rw
>    - f2fs_iomap_begin
>     - f2fs_map_blocks
>      - __allocate_data_block
>       - allocated blkaddr #x
>        - iomap_dio_submit_bio
> 				- f2fs_file_read_iter
> 				 - filemap_read
> 				  - f2fs_read_data_folio
> 				   - f2fs_mpage_readpages
> 				    - f2fs_map_blocks
> 				     : get blkaddr #x
> 				    - f2fs_submit_read_bio
> 				IRQ
> 				- f2fs_read_end_io
> 				 : read IO on blkaddr #x complete
> IRQ
> - iomap_dio_bio_end_io
>  : direct write IO on blkaddr #x complete
> 
Looks like every COW filesystem would meet this situation. What's the solution
of other FS?
> This patch introduces a new per-inode i_opu_rwsem lock to avoid
> such race condition.
>