f2fs: fix a deadlock in fsync

This patch fixes a deadlock bug that occurs quite often when there are
concurrent write and fsync on a same file.

Following is the simplified call trace when tasks get hung.

fsync thread:
- f2fs_sync_file
 ...
 - f2fs_write_data_pages
 ...
  - update_extent_cache
  ...
   - update_inode
    - wait_on_page_writeback

bdi writeback thread
- __writeback_single_inode
 - f2fs_write_data_pages
  - mutex_lock(sbi->writepages)

The deadlock happens when the fsync thread waits on a inode page that has
been added to the f2fs' cached bio sbi->bio[NODE], and unfortunately,
no one else could be able to submit the cached bio to block layer for
writeback. This is because the fsync thread already hold a sbi->fs_lock and
the sbi->writepages lock, causing the bdi thread being blocked when attempt
to write data pages for the same inode. At the same time, f2fs_gc thread
does not notice the situation and could not help. Even the sync syscall
gets blocked.

To fix it, we could submit the cached bio first before waiting on a inode page
that is being written back.

Signed-off-by: Jin Xu <jinuxstyle@gmail.com>
[Jaegeuk Kim: add more cases to use f2fs_wait_on_page_writeback]
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

1 files changed, 2 insertions, 6 deletions

diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
index d286d8b..e6b3ffd 100644
--- a/fs/f2fs/gc.c
+++ b/fs/f2fs/gc.c
@@ -422,8 +422,7 @@ next_step:
 
 		/* set page dirty and write it */
 		if (gc_type == FG_GC) {
-			f2fs_submit_bio(sbi, NODE, true);
-			wait_on_page_writeback(node_page);
+			f2fs_wait_on_page_writeback(node_page, NODE, true);
 			set_page_dirty(node_page);
 		} else {
 			if (!PageWriteback(node_page))
@@ -523,10 +522,7 @@ static void move_data_page(struct inode *inode, struct page *page, int gc_type)
 	} else {
 		struct f2fs_sb_info *sbi = F2FS_SB(inode->i_sb);
 
-		if (PageWriteback(page)) {
-			f2fs_submit_bio(sbi, DATA, true);
-			wait_on_page_writeback(page);
-		}
+		f2fs_wait_on_page_writeback(page, DATA, true);
 
 		if (clear_page_dirty_for_io(page) &&
 			S_ISDIR(inode->i_mode)) {