f2fs: fix to avoid potential deadlock

[ Upstream commit 3441509 ]

Quoted from Jing Xia's report, there is a potential deadlock may happen
between kworker and checkpoint as below:

[T:writeback]				[T:checkpoint]
- wb_writeback
 - blk_start_plug
bio contains NodeA was plugged in writeback threads
					- do_writepages  -- sync write inodeB, inc wb_sync_req[DATA]
					 - f2fs_write_data_pages
					  - f2fs_write_single_data_page -- write last dirty page
					   - f2fs_do_write_data_page
					    - set_page_writeback  -- clear page dirty flag and
					    PAGECACHE_TAG_DIRTY tag in radix tree
					    - f2fs_outplace_write_data
					     - f2fs_update_data_blkaddr
					      - f2fs_wait_on_page_writeback -- wait NodeA to writeback here
					   - inode_dec_dirty_pages
 - writeback_sb_inodes
  - writeback_single_inode
   - do_writepages
    - f2fs_write_data_pages -- skip writepages due to wb_sync_req[DATA]
     - wbc->pages_skipped += get_dirty_pages() -- PAGECACHE_TAG_DIRTY is not set but get_dirty_pages() returns one
  - requeue_inode -- requeue inode to wb->b_dirty queue due to non-zero.pages_skipped
 - blk_finish_plug

Let's try to avoid deadlock condition by forcing unplugging previous bio via
blk_finish_plug(current->plug) once we'v skipped writeback in writepages()
due to valid sbi->wb_sync_req[DATA/NODE].

Fixes: 687de7f ("f2fs: avoid IO split due to mixed WB_SYNC_ALL and WB_SYNC_NONE")
Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
Signed-off-by: Jing Xia <jing.xia@unisoc.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

fs/f2fs/data.c

@@ -3163,8 +3163,12 @@ static int __f2fs_write_data_pages(struct address_space *mapping,
 	/* to avoid spliting IOs due to mixed WB_SYNC_ALL and WB_SYNC_NONE */
 	if (wbc->sync_mode == WB_SYNC_ALL)
 		atomic_inc(&sbi->wb_sync_req[DATA]);
-	else if (atomic_read(&sbi->wb_sync_req[DATA]))
+	else if (atomic_read(&sbi->wb_sync_req[DATA])) {
+		/* to avoid potential deadlock */
+		if (current->plug)
+			blk_finish_plug(current->plug);
 		goto skip_write;
+	}
 
 	if (__should_serialize_io(inode, wbc)) {
 		mutex_lock(&sbi->writepages);

fs/f2fs/node.c

@@ -2111,8 +2111,12 @@ static int f2fs_write_node_pages(struct address_space *mapping,
 
 	if (wbc->sync_mode == WB_SYNC_ALL)
 		atomic_inc(&sbi->wb_sync_req[NODE]);
-	else if (atomic_read(&sbi->wb_sync_req[NODE]))
+	else if (atomic_read(&sbi->wb_sync_req[NODE])) {
+		/* to avoid potential deadlock */
+		if (current->plug)
+			blk_finish_plug(current->plug);
 		goto skip_write;
+	}
 
 	trace_f2fs_writepages(mapping->host, wbc, NODE);