f2fs: fix to avoid deadloop if data_flush is on

As Hagbard Celine reported:

[  615.697824] INFO: task kworker/u16:5:344 blocked for more than 120 seconds.
[  615.697825]       Not tainted 5.0.15-gentoo-f2fslog #4
[  615.697826] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
disables this message.
[  615.697827] kworker/u16:5   D    0   344      2 0x80000000
[  615.697831] Workqueue: writeback wb_workfn (flush-259:0)
[  615.697832] Call Trace:
[  615.697836]  ? __schedule+0x2c5/0x8b0
[  615.697839]  schedule+0x32/0x80
[  615.697841]  schedule_preempt_disabled+0x14/0x20
[  615.697842]  __mutex_lock.isra.8+0x2ba/0x4d0
[  615.697845]  ? log_store+0xf5/0x260
[  615.697848]  f2fs_write_data_pages+0x133/0x320
[  615.697851]  ? trace_hardirqs_on+0x2c/0xe0
[  615.697854]  do_writepages+0x41/0xd0
[  615.697857]  __filemap_fdatawrite_range+0x81/0xb0
[  615.697859]  f2fs_sync_dirty_inodes+0x1dd/0x200
[  615.697861]  f2fs_balance_fs_bg+0x2a7/0x2c0
[  615.697863]  ? up_read+0x5/0x20
[  615.697865]  ? f2fs_do_write_data_page+0x2cb/0x940
[  615.697867]  f2fs_balance_fs+0xe5/0x2c0
[  615.697869]  __write_data_page+0x1c8/0x6e0
[  615.697873]  f2fs_write_cache_pages+0x1e0/0x450
[  615.697878]  f2fs_write_data_pages+0x14b/0x320
[  615.697880]  ? trace_hardirqs_on+0x2c/0xe0
[  615.697883]  do_writepages+0x41/0xd0
[  615.697885]  __filemap_fdatawrite_range+0x81/0xb0
[  615.697887]  f2fs_sync_dirty_inodes+0x1dd/0x200
[  615.697889]  f2fs_balance_fs_bg+0x2a7/0x2c0
[  615.697891]  f2fs_write_node_pages+0x51/0x220
[  615.697894]  do_writepages+0x41/0xd0
[  615.697897]  __writeback_single_inode+0x3d/0x3d0
[  615.697899]  writeback_sb_inodes+0x1e8/0x410
[  615.697902]  __writeback_inodes_wb+0x5d/0xb0
[  615.697904]  wb_writeback+0x28f/0x340
[  615.697906]  ? cpumask_next+0x16/0x20
[  615.697908]  wb_workfn+0x33e/0x420
[  615.697911]  process_one_work+0x1a1/0x3d0
[  615.697913]  worker_thread+0x30/0x380
[  615.697915]  ? process_one_work+0x3d0/0x3d0
[  615.697916]  kthread+0x116/0x130
[  615.697918]  ? kthread_create_worker_on_cpu+0x70/0x70
[  615.697921]  ret_from_fork+0x3a/0x50

There is still deadloop in below condition:

d A
- do_writepages
 - f2fs_write_node_pages
  - f2fs_balance_fs_bg
   - f2fs_sync_dirty_inodes
    - f2fs_write_cache_pages
     - mutex_lock(&sbi->writepages)	-- lock once
     - __write_data_page
      - f2fs_balance_fs_bg
       - f2fs_sync_dirty_inodes
        - f2fs_write_data_pages
         - mutex_lock(&sbi->writepages)	-- lock again

Thread A			Thread B
- do_writepages
 - f2fs_write_node_pages
  - f2fs_balance_fs_bg
   - f2fs_sync_dirty_inodes
    - .cp_task = current
				- f2fs_sync_dirty_inodes
				 - .cp_task = current
				 - filemap_fdatawrite
				 - .cp_task = NULL
    - filemap_fdatawrite
     - f2fs_write_cache_pages
      - enter f2fs_balance_fs_bg since .cp_task is NULL
    - .cp_task = NULL

Change as below to avoid this:
- add condition to avoid holding .writepages mutex lock in path
of data flush
- introduce mutex lock sbi.flush_lock to exclude concurrent data
flush in background.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -2332,6 +2332,9 @@ static inline bool __should_serialize_io(struct inode *inode,
 		return false;
 	if (IS_NOQUOTA(inode))
 		return false;
+	/* to avoid deadlock in path of data flush */
+	if (F2FS_I(inode)->cp_task)
+		return false;
 	if (wbc->sync_mode != WB_SYNC_ALL)
 		return true;
 	if (get_dirty_pages(inode) >= SM_I(F2FS_I_SB(inode))->min_seq_blocks)

fs/f2fs/f2fs.h

@@ -1209,6 +1209,7 @@ struct f2fs_sb_info {
 	/* for inode management */
 	struct list_head inode_list[NR_INODE_TYPE];	/* dirty inode list */
 	spinlock_t inode_lock[NR_INODE_TYPE];	/* for dirty inode list lock */
+	struct mutex flush_lock;		/* for flush exclusion */
 
 	/* for extent tree cache */
 	struct radix_tree_root extent_tree_root;/* cache extent cache entries */

fs/f2fs/segment.c

@@ -546,9 +546,13 @@ void f2fs_balance_fs_bg(struct f2fs_sb_info *sbi)
 		if (test_opt(sbi, DATA_FLUSH)) {
 			struct blk_plug plug;
 
+			mutex_lock(&sbi->flush_lock);
+
 			blk_start_plug(&plug);
 			f2fs_sync_dirty_inodes(sbi, FILE_INODE);
 			blk_finish_plug(&plug);
+
+			mutex_unlock(&sbi->flush_lock);
 		}
 		f2fs_sync_fs(sbi->sb, true);
 		stat_inc_bg_cp_count(sbi->stat_info);

fs/f2fs/super.c

@@ -3293,6 +3293,7 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 		INIT_LIST_HEAD(&sbi->inode_list[i]);
 		spin_lock_init(&sbi->inode_lock[i]);
 	}
+	mutex_init(&sbi->flush_lock);
 
 	f2fs_init_extent_cache_info(sbi);