Re: [RFC PATCH] kernfs: release kernfs_mutex before the inode allocation

[Minchan Kim <minchan@kernel.org>]

Hi Tejun,

On Wed, Nov 17, 2021 at 11:45:46AM -1000, Tejun Heo wrote:
> Hello,
> 
> On Tue, Nov 16, 2021 at 11:27:56PM -0800, Minchan Kim wrote:
> > A app launching involves dma_buf exports which creates kobject
> > and add it to the kernfs with down_write - kernfs_add_one.
> > 
> > At the same time in other CPU, a random process was accessing
> > sysfs and the kernfs_iop_lookup was already hoding the kernfs_rwsem
> > and ran under direct reclaim patch due to alloc_inode in
> > kerfs_get_inode.
> > 
> > Therefore, the app is stuck on the lock and lose frames so enduser
> > sees the jank.
> 
> So, one really low hanging fruit here would be using a separate rwsem per
> superblock. Nothing needs synchronization across different users of kernfs
> and the locking is shared just because nobody bothered to separate them out
> while generalizing it from sysfs.

That's really what I wanted but had a question whether we can access
superblock from the kernfs_node all the time since there are some
functions to access the kernfs_rwsem without ionde, sb context.

Is it doable to get the superblock from the kernfs_node all the time?