linux-kernel - Re: [PATCH v2 1/2] cxgb4: sched: Use refcount

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Message-ID: <CANhBUQ2C3OfkC6qDL9=hhXq=C-OMHUwaL7EaMbagVRTt=rc00A@mail.gmail.com>
Date:   Fri, 2 Aug 2019 23:10:39 +0800
From:   Chuhong Yuan <hslester96@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Vishal Kulkarni <vishal@...lsio.com>,
        "David S . Miller" <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] cxgb4: sched: Use refcount_t for refcount

Willem de Bruijn <willemdebruijn.kernel@...il.com> 于2019年8月2日周五 下午10:53写道：
>
> On Fri, Aug 2, 2019 at 10:27 AM Chuhong Yuan <hslester96@...il.com> wrote:
> >
> > Willem de Bruijn <willemdebruijn.kernel@...il.com> 于2019年8月2日周五 下午9:40写道：
> > >
> > > On Fri, Aug 2, 2019 at 4:36 AM Chuhong Yuan <hslester96@...il.com> wrote:
> > > >
> > > > refcount_t is better for reference counters since its
> > > > implementation can prevent overflows.
> > > > So convert atomic_t ref counters to refcount_t.
> > > >
> > > > Signed-off-by: Chuhong Yuan <hslester96@...il.com>
> > > > ---
> > > > Changes in v2:
> > > >   - Convert refcount from 0-base to 1-base.
> > >
> > > This changes the initial value from 0 to 1, but does not change the
> > > release condition. So this introduces an accounting bug?
> >
> > I have noticed this problem and have checked other files which use refcount_t.
> > I find although the refcounts are 1-based, they still use
> > refcount_dec_and_test()
> > to check whether the resource should be released.
> > One example is drivers/char/mspec.c.
> > Therefore I think this is okay and do not change the release condition.
>
> Indeed it is fine to use refcount_t with a model where the initial
> allocation already accounts for the first reference and thus
> initializes with refcount_set(.., 1).
>
> But it is not correct to just change a previously zero initialization
> to one. As now an extra refcount_dec will be needed to release state.
> But the rest of the code has not changed, so this extra decrement will
> not happen.
>
> For a correct conversion, see for instance commits
>
>   commit db5bce32fbe19f0c7482fb5a40a33178bbe7b11b
>   net: prepare (struct ubuf_info)->refcnt conversion
>
> and
>
>   commit c1d1b437816f0afa99202be3cb650c9d174667bc
>   net: convert (struct ubuf_info)->refcnt to refcount_t
>
> The second makes a search-and-replace style API change like your
> patches (though also notice the additional required #include).
>

Thanks for your examples!
I will fix the #include in those no base changed patches.

> But the other patch is needed first to change both the initial
> atomic_set *and* at least one atomic_inc, to maintain the same
> reference count over the object's lifetime.
>
> That change requires understanding of the object's lifecycle, so I
> suggest only making those changes when aware of that whole data path.

I think I had better focus on the 1-based cases first.