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

From: Fox Chen <foxhlchen@gmail.com>
To: Ian Kent <raven@themaw.net>
Cc: akpm@linux-foundation.org, dhowells@redhat.com,
	Greg KH <gregkh@linuxfoundation.org>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	miklos@szeredi.hu, ricklind@linux.vnet.ibm.com,
	sfr@canb.auug.org.au, Tejun Heo <tj@kernel.org>,
	viro@zeniv.linux.org.uk
Subject: Re: [PATCH v2 0/6] kernfs: proposed locking and concurrency improvement
Date: Mon, 14 Dec 2020 14:14:13 +0800	[thread overview]
Message-ID: <CAC2o3DJdHuQxY7Rn5uXUprS7i8ri1qB=wOUM2rdZkWt4yJHv1w@mail.gmail.com> (raw)
In-Reply-To: <bde0b6c32f2b055c1ad1401b45c4adf61aab6876.camel@themaw.net>

On Sun, Dec 13, 2020 at 11:46 AM Ian Kent <raven@themaw.net> wrote:
>
> 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.

Oh, understood. I was asking also because lock on kn->attr_mutex drags
concurrent performance.

> 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().

You meant taking write lock of kernfs_rwsem for kernfs_refresh_inode()??
It may be a lot slower in my benchmark, let me test it.

> 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
>

thanks,
fox