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

[Minchan Kim <minchan@kernel.org>]

On Wed, Nov 17, 2021 at 12:23:55PM -1000, Tejun Heo wrote:
> Hello,
> 
> 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:

Thanks for the suggestion, Tejun.

I found kernfs_root and it seems like to return kernfs_root from kernfs_node.
If it's true all the case, we would put the rwsem in kernfs_root and change
would be straightforward. Do you see any problem?

> 
> 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.
> 
> Thanks.
> 
> -- 
> tejun