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Date:   Mon, 12 Aug 2019 16:00:19 -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
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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.

--b.