f2fs: fix to access nullified flush_cmd_control pointer

f2fs_sync_file()             remount_ro
 - f2fs_readonly
                               - destroy_flush_cmd_control
 - f2fs_issue_flush
   - no fcc pointer!

So, this patch doesn't free fcc in this case, but just stop its kernel thread
which sends flush commands.
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -2103,7 +2103,7 @@ void f2fs_balance_fs(struct f2fs_sb_info *, bool);
 void f2fs_balance_fs_bg(struct f2fs_sb_info *);
 int f2fs_issue_flush(struct f2fs_sb_info *);
 int create_flush_cmd_control(struct f2fs_sb_info *);
-void destroy_flush_cmd_control(struct f2fs_sb_info *);
+void destroy_flush_cmd_control(struct f2fs_sb_info *, bool);
 void invalidate_blocks(struct f2fs_sb_info *, block_t);
 bool is_checkpointed_data(struct f2fs_sb_info *, block_t);
 void refresh_sit_entry(struct f2fs_sb_info *, block_t, block_t);

fs/f2fs/segment.c

@@ -489,8 +489,13 @@ int f2fs_issue_flush(struct f2fs_sb_info *sbi)
 	if (!fcc->dispatch_list)
 		wake_up(&fcc->flush_wait_queue);
 
-	wait_for_completion(&cmd.wait);
-	atomic_dec(&fcc->submit_flush);
+	if (fcc->f2fs_issue_flush) {
+		wait_for_completion(&cmd.wait);
+		atomic_dec(&fcc->submit_flush);
+	} else {
+		llist_del_all(&fcc->issue_list);
+		atomic_set(&fcc->submit_flush, 0);
+	}
 
 	return cmd.ret;
 }
@@ -501,6 +506,11 @@ int create_flush_cmd_control(struct f2fs_sb_info *sbi)
 	struct flush_cmd_control *fcc;
 	int err = 0;
 
+	if (SM_I(sbi)->cmd_control_info) {
+		fcc = SM_I(sbi)->cmd_control_info;
+		goto init_thread;
+	}
+
 	fcc = kzalloc(sizeof(struct flush_cmd_control), GFP_KERNEL);
 	if (!fcc)
 		return -ENOMEM;
@@ -508,6 +518,7 @@ int create_flush_cmd_control(struct f2fs_sb_info *sbi)
 	init_waitqueue_head(&fcc->flush_wait_queue);
 	init_llist_head(&fcc->issue_list);
 	SM_I(sbi)->cmd_control_info = fcc;
+init_thread:
 	fcc->f2fs_issue_flush = kthread_run(issue_flush_thread, sbi,
 				"f2fs_flush-%u:%u", MAJOR(dev), MINOR(dev));
 	if (IS_ERR(fcc->f2fs_issue_flush)) {
@@ -520,14 +531,20 @@ int create_flush_cmd_control(struct f2fs_sb_info *sbi)
 	return err;
 }
 
-void destroy_flush_cmd_control(struct f2fs_sb_info *sbi)
+void destroy_flush_cmd_control(struct f2fs_sb_info *sbi, bool free)
 {
 	struct flush_cmd_control *fcc = SM_I(sbi)->cmd_control_info;
 
-	if (fcc && fcc->f2fs_issue_flush)
-		kthread_stop(fcc->f2fs_issue_flush);
-	kfree(fcc);
-	SM_I(sbi)->cmd_control_info = NULL;
+	if (fcc && fcc->f2fs_issue_flush) {
+		struct task_struct *flush_thread = fcc->f2fs_issue_flush;
+
+		fcc->f2fs_issue_flush = NULL;
+		kthread_stop(flush_thread);
+	}
+	if (free) {
+		kfree(fcc);
+		SM_I(sbi)->cmd_control_info = NULL;
+	}
 }
 
 static void __locate_dirty_segment(struct f2fs_sb_info *sbi, unsigned int segno,
@@ -2738,7 +2755,7 @@ void destroy_segment_manager(struct f2fs_sb_info *sbi)
 
 	if (!sm_info)
 		return;
-	destroy_flush_cmd_control(sbi);
+	destroy_flush_cmd_control(sbi, true);
 	destroy_dirty_segmap(sbi);
 	destroy_curseg(sbi);
 	destroy_free_segmap(sbi);

fs/f2fs/super.c

@@ -1102,8 +1102,9 @@ static int f2fs_remount(struct super_block *sb, int *flags, char *data)
 	 * or if flush_merge is not passed in mount option.
 	 */
 	if ((*flags & MS_RDONLY) || !test_opt(sbi, FLUSH_MERGE)) {
-		destroy_flush_cmd_control(sbi);
-	} else if (!SM_I(sbi)->cmd_control_info) {
+		clear_opt(sbi, FLUSH_MERGE);
+		destroy_flush_cmd_control(sbi, false);
+	} else {
 		err = create_flush_cmd_control(sbi);
 		if (err)
 			goto restore_gc;