linux-kernel - Re: [PATCH 3/5] fs, xfs: convert xlog_ticket.t_ref from atomic_t to refcount

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Message-ID: <CAGXu5jK3x+cPe4pccv0Q5W+9eDxu7n3Enih-TVRkysmwDwzGCA@mail.gmail.com>
Date:   Wed, 8 Mar 2017 12:06:49 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Elena Reshetova <elena.reshetova@...el.com>,
        darrick.wong@...cle.com, LKML <linux-kernel@...r.kernel.org>,
        linux-xfs@...r.kernel.org, Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>
Subject: Re: [PATCH 3/5] fs, xfs: convert xlog_ticket.t_ref from atomic_t to refcount_t

On Wed, Mar 8, 2017 at 7:50 AM, Christoph Hellwig <hch@...radead.org> wrote:
>> -     ASSERT(atomic_read(&ticket->t_ref) > 0);
>> -     atomic_inc(&ticket->t_ref);
>> +     ASSERT(refcount_read(&ticket->t_ref) > 0);
>> +     refcount_inc(&ticket->t_ref);
>
> With strict refcount semantics refcount_inc should check that
> the count is larger than 0, otherwise we'd need to use
> recount_inc_not_zero or whatever you're going to call it.
>
> Is that something the recount code does / could do?

Yes, refcount_inc() will not increment from 0 and WARNs. It looks like
xfs's ASSERT is also a warn (though with XFS-specific formatting), so
perhaps the ASSERT could be dropped? IIUC, Elena's approach to these
changes was to be conservative about removing the existing checks.

-Kees

-- 
Kees Cook
Pixel Security