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

[Chao Yu]

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

In LFS mode, if there is inflight dio, let's force read to buffered
IO, this policy won't cover all race cases, however it is a tradeoff
which avoids abusing lock around IO paths.

Fixes: f847c699cff3 ("f2fs: allow out-place-update for direct IO in LFS mode")
Signed-off-by: Chao Yu <chao@xxxxxxxxxx>
---
 fs/f2fs/file.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 278573974db4..866f1a34e92b 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -882,6 +882,10 @@ static bool f2fs_force_buffered_io(struct inode *inode, int rw)
 		return true;
 	if (is_sbi_flag_set(sbi, SBI_CP_DISABLED))
 		return true;
+	/* In LFS mode, if there is inflight dio, force read to buffered IO */
+	if (rw == READ && f2fs_lfs_mode(sbi) &&
+			atomic_read(&inode->i_dio_count))
+		return false;
 
 	return false;
 }
-- 
2.40.1