[f2fs-dev] [PATCH v3] f2fs: fix to avoid racing in between read and OPU dio write

[Chao Yu <chao@kernel.org>]

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 wait for its completion,
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@kernel.org>
---
v3:
- wait for direct IO's completion before read IO.
 fs/f2fs/file.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 9e171e138943..76a6043caf27 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -4608,6 +4608,10 @@ static ssize_t f2fs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 		f2fs_trace_rw_file_path(iocb->ki_filp, iocb->ki_pos,
 					iov_iter_count(to), READ);
 
+	/* In LFS mode, if there is inflight dio, wait for its completion */
+	if (f2fs_lfs_mode(F2FS_I_SB(inode)))
+		inode_dio_wait(inode);
+
 	if (f2fs_should_use_dio(inode, iocb, to)) {
 		ret = f2fs_dio_read_iter(iocb, to);
 	} else {
-- 
2.40.1



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