Re: [PATCH] ext4: remove abnormal set for I_DATA_SEM subclass

["Theodore Y. Ts'o" <tytso@mit.edu>]

On Mon, Jul 09, 2018 at 04:58:28PM +0900, Junil Lee wrote:
> The -EBUSY return value of dquot_enable() function means that just
> want to update flags. If some users make a duplicate request to update
> flags, lockdep could catch the false positive casued by needing to
> allocate a quota block from inside ext4_map_blocks(), while holding
> i_data_sem for a data inode. This results in this complaint:
> 
>        CPU0                    CPU1
>        ----                    ----
>   lock(&s->s_dquot.dqio_mutex);
>                                lock(&ei->i_data_sem);
>                                lock(&s->s_dquot.dqio_mutex);
>   lock(&ei->i_data_sem);

How does this happen in practice?  The function ext4_quota_enable() is
only called by ext4_enable_quotas(), and I don't see the code path
where this would happen.  And if it does it would be resulting an
EXT4-fs warning message getting printing indicating that a failure to
enable quotas with an error of EBUSY.  So how does this happen that
"users would make a duplicate request to update flags"?

       	     	    	      	      	 	- Ted