linux-kernel - Re: [RFC PATCH] kernfs: release kernfs

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Message-ID: <YZWA+8B1xQOKCMnS@slm.duckdns.org>
Date:   Wed, 17 Nov 2021 12:23:55 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] kernfs: release kernfs_mutex before the inode
 allocation

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:

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