From: Jiayuan Chen <[email protected]> KCSAN reports a data race when concurrent readers access the same struct file:
BUG: KCSAN: data-race in filemap_read / filemap_splice_read write to 0xffff88811a6f8228 of 8 bytes by task 10061 on cpu 0: filemap_splice_read+0x523/0x780 mm/filemap.c:3125 ... write to 0xffff88811a6f8228 of 8 bytes by task 10066 on cpu 1: filemap_read+0x98d/0xa10 mm/filemap.c:2873 ... Both filemap_read() and filemap_splice_read() update f_ra.prev_pos without synchronization. This is a benign race since prev_pos is only used as a hint for readahead heuristics in page_cache_sync_ra(), and a stale or torn value merely results in a suboptimal readahead decision, not a correctness issue. Use WRITE_ONCE/READ_ONCE to annotate all accesses to prev_pos across the tree for consistency and silence KCSAN. Reported-by: [email protected] Link: https://syzkaller.appspot.com/bug?extid=6880f676b265dbd42d63 Signed-off-by: Jiayuan Chen <[email protected]> --- fs/ext4/dir.c | 2 +- fs/ntfs3/fsntfs.c | 2 +- include/trace/events/readahead.h | 2 +- mm/filemap.c | 6 +++--- mm/readahead.c | 4 ++-- mm/shmem.c | 2 +- 6 files changed, 9 insertions(+), 9 deletions(-) diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c index 28b2a3deb954..1ddf7acce5ca 100644 --- a/fs/ext4/dir.c +++ b/fs/ext4/dir.c @@ -200,7 +200,7 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx) sb->s_bdev->bd_mapping, &file->f_ra, file, index, 1 << EXT4_SB(sb)->s_min_folio_order); - file->f_ra.prev_pos = (loff_t)index << PAGE_SHIFT; + WRITE_ONCE(file->f_ra.prev_pos, (loff_t)index << PAGE_SHIFT); bh = ext4_bread(NULL, inode, map.m_lblk, 0); if (IS_ERR(bh)) { err = PTR_ERR(bh); diff --git a/fs/ntfs3/fsntfs.c b/fs/ntfs3/fsntfs.c index 0df2aa81d884..d1232fc03c08 100644 --- a/fs/ntfs3/fsntfs.c +++ b/fs/ntfs3/fsntfs.c @@ -1239,7 +1239,7 @@ int ntfs_read_run_nb_ra(struct ntfs_sb_info *sbi, const struct runs_tree *run, if (!ra_has_index(ra, index)) { page_cache_sync_readahead(mapping, ra, NULL, index, 1); - ra->prev_pos = (loff_t)index << PAGE_SHIFT; + WRITE_ONCE(ra->prev_pos, (loff_t)index << PAGE_SHIFT); } } diff --git a/include/trace/events/readahead.h b/include/trace/events/readahead.h index 0997ac5eceab..63d8df6c2983 100644 --- a/include/trace/events/readahead.h +++ b/include/trace/events/readahead.h @@ -101,7 +101,7 @@ DECLARE_EVENT_CLASS(page_cache_ra_op, __entry->async_size = ra->async_size; __entry->ra_pages = ra->ra_pages; __entry->mmap_miss = ra->mmap_miss; - __entry->prev_pos = ra->prev_pos; + __entry->prev_pos = READ_ONCE(ra->prev_pos); __entry->req_count = req_count; ), diff --git a/mm/filemap.c b/mm/filemap.c index 63f256307fdd..d3e2d4b826b9 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2771,7 +2771,7 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter, int i, error = 0; bool writably_mapped; loff_t isize, end_offset; - loff_t last_pos = ra->prev_pos; + loff_t last_pos = READ_ONCE(ra->prev_pos); if (unlikely(iocb->ki_pos < 0)) return -EINVAL; @@ -2870,7 +2870,7 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter, } while (iov_iter_count(iter) && iocb->ki_pos < isize && !error); file_accessed(filp); - ra->prev_pos = last_pos; + WRITE_ONCE(ra->prev_pos, last_pos); return already_read ? already_read : error; } EXPORT_SYMBOL_GPL(filemap_read); @@ -3122,7 +3122,7 @@ ssize_t filemap_splice_read(struct file *in, loff_t *ppos, len -= n; total_spliced += n; *ppos += n; - in->f_ra.prev_pos = *ppos; + WRITE_ONCE(in->f_ra.prev_pos, *ppos); if (pipe_is_full(pipe)) goto out; } diff --git a/mm/readahead.c b/mm/readahead.c index 7b05082c89ea..de49b35b0329 100644 --- a/mm/readahead.c +++ b/mm/readahead.c @@ -142,7 +142,7 @@ void file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping) { ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages; - ra->prev_pos = -1; + WRITE_ONCE(ra->prev_pos, -1); } EXPORT_SYMBOL_GPL(file_ra_state_init); @@ -584,7 +584,7 @@ void page_cache_sync_ra(struct readahead_control *ractl, } max_pages = ractl_max_pages(ractl, req_count); - prev_index = (unsigned long long)ra->prev_pos >> PAGE_SHIFT; + prev_index = (unsigned long long)READ_ONCE(ra->prev_pos) >> PAGE_SHIFT; /* * A start of file, oversized read, or sequential cache miss: * trivial case: (index - prev_index) == 1 diff --git a/mm/shmem.c b/mm/shmem.c index 5e7dcf5bc5d3..03569199baf4 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -3642,7 +3642,7 @@ static ssize_t shmem_file_splice_read(struct file *in, loff_t *ppos, len -= n; total_spliced += n; *ppos += n; - in->f_ra.prev_pos = *ppos; + WRITE_ONCE(in->f_ra.prev_pos, *ppos); if (pipe_is_full(pipe)) break; -- 2.43.0
