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Subject: Re: [PATCH v2 0/6] kernfs: proposed locking and concurrency
 improvement
From:   Ian Kent <raven@themaw.net>
To:     Tejun Heo <tj@kernel.org>
Cc:     Fox Chen <foxhlchen@gmail.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        akpm@linux-foundation.org, dhowells@redhat.com,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        miklos@szeredi.hu, ricklind@linux.vnet.ibm.com,
        sfr@canb.auug.org.au, viro@zeniv.linux.org.uk
Date:   Fri, 18 Dec 2020 15:36:21 +0800
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On Thu, 2020-12-17 at 10:14 -0500, Tejun Heo wrote:
> Hello,
> 
> On Thu, Dec 17, 2020 at 07:48:49PM +0800, Ian Kent wrote:
> > > What could be done is to make the kernfs node attr_mutex
> > > a pointer and dynamically allocate it but even that is too
> > > costly a size addition to the kernfs node structure as
> > > Tejun has said.
> > 
> > I guess the question to ask is, is there really a need to
> > call kernfs_refresh_inode() from functions that are usually
> > reading/checking functions.
> > 
> > Would it be sufficient to refresh the inode in the write/set
> > operations in (if there's any) places where things like
> > setattr_copy() is not already called?
> > 
> > Perhaps GKH or Tejun could comment on this?
> 
> My memory is a bit hazy but invalidations on reads is how sysfs
> namespace is
> implemented, so I don't think there's an easy around that. The only
> thing I
> can think of is embedding the lock into attrs and doing xchg dance
> when
> attaching it.

Sounds like your saying it would be ok to add a lock to the
attrs structure, am I correct?

Assuming it is then, to keep things simple, use two locks.

One global lock for the allocation and an attrs lock for all the
attrs field updates including the kernfs_refresh_inode() update.

The critical section for the global lock could be reduced and it
changed to a spin lock.

In __kernfs_iattrs() we would have something like:

take the allocation lock
do the allocated checks
  assign if existing attrs
  release the allocation lock
  return existing if found
othewise
  release the allocation lock

allocate and initialize attrs

take the allocation lock
check if someone beat us to it
  free and grab exiting attrs
otherwise
  assign the new attrs
release the allocation lock
return attrs

Add a spinlock to the attrs struct and use it everywhere for
field updates.

Am I on the right track or can you see problems with this?

Ian