linux-kernel - Re: [PATCH v2 0/6] kernfs: proposed locking and concurrency improvement

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Date:   Sun, 13 Dec 2020 11:46:13 +0800
From:   Ian Kent <raven@...maw.net>
To:     Fox Chen <foxhlchen@...il.com>
Cc:     akpm@...ux-foundation.org, dhowells@...hat.com,
        gregkh@...uxfoundation.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, miklos@...redi.hu,
        ricklind@...ux.vnet.ibm.com, sfr@...b.auug.org.au, tj@...nel.org,
        viro@...IV.linux.org.uk
Subject: Re: [PATCH v2 0/6] kernfs: proposed locking and concurrency
 improvement

On Fri, 2020-12-11 at 10:17 +0800, Ian Kent wrote:
> On Fri, 2020-12-11 at 10:01 +0800, Ian Kent wrote:
> > > For the patches, there is a mutex_lock in kn->attr_mutex, as
> > > Tejun
> > > mentioned here 
> > > (https://lore.kernel.org/lkml/X8fe0cmu+aq1gi7O@mtj.duckdns.org/),
> > > maybe a global 
> > > rwsem for kn->iattr will be better??
> > 
> > I wasn't sure about that, IIRC a spin lock could be used around the
> > initial check and checked again at the end which would probably
> > have
> > been much faster but much less conservative and a bit more ugly so
> > I just went the conservative path since there was so much change
> > already.
> 
> Sorry, I hadn't looked at Tejun's reply yet and TBH didn't remember
> it.
> 
> Based on what Tejun said it sounds like that needs work.

Those attribute handling patches were meant to allow taking the rw
sem read lock instead of the write lock for kernfs_refresh_inode()
updates, with the added locking to protect the inode attributes
update since it's called from the VFS both with and without the
inode lock.

Looking around it looks like kernfs_iattrs() is called from multiple
places without a node database lock at all.

I'm thinking that, to keep my proposed change straight forward
and on topic, I should just leave kernfs_refresh_inode() taking
the node db write lock for now and consider the attributes handling
as a separate change. Once that's done we could reconsider what's
needed to use the node db read lock in kernfs_refresh_inode().

It will reduce the effectiveness of the series but it would make
this change much more complicated, and is somewhat off-topic, and
could hamper the chances of reviewers spotting problem with it.

Ian