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

From: Tejun Heo
Date: Wed Nov 17 2021 - 17:24:08 EST


On Wed, Nov 17, 2021 at 02:13:35PM -0800, Minchan Kim wrote:
> > 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?

Ah, right, kernfs_node doesn't point back to kernfs_root. I guess it can go
one of three ways:

a. Follow parent until root kernfs_node and make that guy point to
kernfs_root through its parent field. This isn't great but the hotter
paths all have sb / inode already, I think, so if we do this only in the
really cold paths, it likely isn't too bad.

b. Change the interface so that the callers have to provide kernfs_root. I
don't think this is gonna be a huge problem. There are a few users of
kernfs and they always know their roots.

c. Add a field to kernfs_node so that we can always find kernfs_root.

I think b is likely the cheapest && cleanest.