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Date:   Fri, 18 Dec 2020 19:21:10 +0800
From:   Ian Kent <raven@...maw.net>
To:     Fox Chen <foxhlchen@...il.com>
Cc:     Tejun Heo <tj@...nel.org>, Greg KH <gregkh@...uxfoundation.org>,
        akpm@...ux-foundation.org, dhowells@...hat.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        miklos@...redi.hu, ricklind@...ux.vnet.ibm.com,
        sfr@...b.auug.org.au, viro@...iv.linux.org.uk
Subject: Re: [PATCH v2 0/6] kernfs: proposed locking and concurrency
 improvement

On Fri, 2020-12-18 at 16:01 +0800, Fox Chen wrote:
> On Fri, Dec 18, 2020 at 3:36 PM Ian Kent <raven@...maw.net> wrote:
> > 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
> > 
> 
> umm, we update the inode in kernfs_refresh_inode, right??  So I guess
> the problem is how can we protect the inode when kernfs_refresh_inode
> is called, not the attrs??

But the attrs (which is what's copied from) were protected by the
mutex lock (IIUC) so dealing with the inode attributes implies
dealing with the kernfs node attrs too.

For example in kernfs_iop_setattr() the call to setattr_copy() copies
the node attrs to the inode under the same mutex lock. So, if a read
lock is used the copy in kernfs_refresh_inode() is no longer protected,
it needs to be protected in a different way.

Ian

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