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

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Date:   Fri, 18 Dec 2020 16:01:14 +0800
From:   Fox Chen <foxhlchen@...il.com>
To:     Ian Kent <raven@...maw.net>
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, 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??


thanks,
fox