Hi Andrea,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on linus/master]
[also build test ERROR on v5.0 next-20190306]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Andrea-Righi/blkcg-prevent-priority-inversion-problem-during-sync/20190310-020543
config: riscv-tinyconfig (attached as .config)
compiler: riscv64-linux-gcc (GCC) 8.2.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        GCC_VERSION=8.2.0 make.cross ARCH=riscv 

All errors (new ones prefixed by >>):

   fs/sync.c: In function 'sync_fs_one_sb':
>> fs/sync.c:83:3: error: implicit declaration of function 'blkcg_start_wb_wait_on_bdi' [-Werror=implicit-function-declaration]
      blkcg_start_wb_wait_on_bdi(bdi);
      ^~~~~~~~~~~~~~~~~~~~~~~~~~
>> fs/sync.c:85:3: error: implicit declaration of function 'blkcg_stop_wb_wait_on_bdi' [-Werror=implicit-function-declaration]
      blkcg_stop_wb_wait_on_bdi(bdi);
      ^~~~~~~~~~~~~~~~~~~~~~~~~
   cc1: some warnings being treated as errors

vim +/blkcg_start_wb_wait_on_bdi +83 fs/sync.c

    77	
    78	static void sync_fs_one_sb(struct super_block *sb, void *arg)
    79	{
    80		struct backing_dev_info *bdi = sb->s_bdi;
    81	
    82		if (!sb_rdonly(sb) && sb->s_op->sync_fs) {
  > 83			blkcg_start_wb_wait_on_bdi(bdi);
    84			sb->s_op->sync_fs(sb, *(int *)arg);
  > 85			blkcg_stop_wb_wait_on_bdi(bdi);
    86		}
    87	}
    88	

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