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Message-ID: <20161207132457.GG3107@twins.programming.kicks-ass.net>
Date: Wed, 7 Dec 2016 14:24:57 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: David Windsor <dwindsor@...il.com>
Cc: Boqun Feng <boqun.feng@...il.com>, Kees Cook <keescook@...omium.org>,
"Reshetova, Elena" <elena.reshetova@...el.com>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
Greg KH <gregkh@...uxfoundation.org>,
"will.deacon@....com" <will.deacon@....com>,
Hans Liljestrand <ishkamiel@...il.com>,
"aik@...abs.ru" <aik@...abs.ru>,
"david@...son.dropbear.id.au" <david@...son.dropbear.id.au>
Subject: Re: Conversion from atomic_t to refcount_t: summary of issues
On Fri, Dec 02, 2016 at 03:25:42PM -0500, David Windsor wrote:
> On Thu, Dec 1, 2016 at 8:17 PM, Boqun Feng <boqun.feng@...il.com> wrote:
> > So we currently don't have a clear semantics for stats_t, do we?
>
> We had a discussion about the stats_t API in another thread. We
> agreed upon add(), sub(), inc(), dec(), read() and set().
>
> > What kind of atomic_t should be replaced with stats_t?
>
> stats_t is used for those cases in which an atomic variable is
> required, but the overflow of this variable isn't of much concern.
> Typically, these types of variables are counters of some kind (rx/tx
> counts, etc), but not always. Perhaps "stats_t" isn't the best type
> name. We actually used "atomic_wrap_t" in previous iterations.
And atomic_wrap_t is a horrid trainwreck. Please as to explain the
semantics of atomic_wrap_cmpxchg(). How does wrapping apply to something
that doesn't do sign bits.
> > In the link David pointed out, there are a few cases where a
> > stats_t is put on a correctness-related variable. I don't think
> > that's a good place to use stats_t.
> >
>
> Yeah, I just grabbed a few examples I noted during my stats_t
> conversion work. The drivers/ tree is littered with stats_t
> instances.
Not sure how to respond to this, if you're converting all that to
stats_t then you're doing it wrong.
Most of what you showed should very emphatically not be stats_t.
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