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Date:   Mon, 12 Aug 2019 16:00:36 -0400
From:   "J. Bruce Fields" <bfields@redhat.com>
To:     Olga Kornievskaia <olga.kornievskaia@gmail.com>
Cc:     linux-nfs <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH v5 5/9] NFSD add COPY_NOTIFY operation
Message-ID: <20190812200036.GC29812@parsley.fieldses.org>
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On Mon, Aug 12, 2019 at 04:00:19PM -0400, J. Bruce Fields wrote:
> On Mon, Aug 12, 2019 at 03:16:47PM -0400, Olga Kornievskaia wrote:
> > On Mon, Aug 12, 2019 at 12:19 PM Olga Kornievskaia
> > <olga.kornievskaia@gmail.com> wrote:
> > > While this passes my testing, in theory this allows for the race that
> > > we get the copy notify size but then offload_cancel arrive and change
> > > the value. Then refcount_sub_and test_check would have an incorrect
> > > value (can subtract larger than an actual reference count). I have no
> > > solution for that as there is no refcount_sub_and_lock() that will
> > > allow to decrement by a multiple under a lock. Thoughts?
> > 
> > I tried not to use the client's cl_lock but instead use a specific
> > lock to protect the copy notifies stateid on the stateid list. But
> > since stateid's reference counter (sc_count) is protected by it, I
> > think by getting rid of the special lock and using cl_lock will solve
> > the problem of coordinating access between the sc_count and the
> > copy_notify stateid list. Are the any problems with using such a big
> > lock?
> 
> Probably not.  But it can be confusing when a single lock is used for
> several different things.  A comment explaining why you need it might
> help.

(By which I mean, a comment in the code.)

--b.