Re: [PATCH 00/11] Introduce Simple atomic and non-atomic counters

From: Joel Fernandes <joel@joelfernandes.org>
To: Kees Cook <keescook@chromium.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>,
	corbet@lwn.net, gregkh@linuxfoundation.org, shuah@kernel.org,
	rafael@kernel.org, johannes@sipsolutions.net, lenb@kernel.org,
	james.morse@arm.com, tony.luck@intel.com, bp@alien8.de,
	arve@android.com, tkjos@android.com, maco@android.com,
	christian@brauner.io, hridya@google.com, surenb@google.com,
	minyard@acm.org, arnd@arndb.de, mchehab@kernel.org,
	rric@kernel.org, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	linux-acpi@vger.kernel.org, devel@driverdev.osuosl.org,
	openipmi-developer@lists.sourceforge.net,
	linux-edac@vger.kernel.org
Subject: Re: [PATCH 00/11] Introduce Simple atomic and non-atomic counters
Date: Mon, 28 Sep 2020 17:17:09 -0400	[thread overview]
Message-ID: <20200928211709.GA2641213@google.com> (raw)
In-Reply-To: <202009281331.444F36A7B@keescook>

On Mon, Sep 28, 2020 at 01:34:31PM -0700, Kees Cook wrote:
> On Sun, Sep 27, 2020 at 07:35:26PM -0400, Joel Fernandes wrote:
> > On Fri, Sep 25, 2020 at 05:47:14PM -0600, Shuah Khan wrote:
> > > This patch series is a result of discussion at the refcount_t BOF
> > > the Linux Plumbers Conference. In this discussion, we identified
> > > a need for looking closely and investigating atomic_t usages in
> > > the kernel when it is used strictly as a counter without it
> > > controlling object lifetimes and state changes.
> > > 
> > > There are a number of atomic_t usages in the kernel where atomic_t api
> > > is used strictly for counting and not for managing object lifetime. In
> > > some cases, atomic_t might not even be needed.
> > >     
> > > The purpose of these counters is twofold: 1. clearly differentiate
> > > atomic_t counters from atomic_t usages that guard object lifetimes,
> > > hence prone to overflow and underflow errors. It allows tools that scan
> > > for underflow and overflow on atomic_t usages to detect overflow and
> > > underflows to scan just the cases that are prone to errors. 2. provides
> > > non-atomic counters for cases where atomic isn't necessary.
> > 
> > Nice series :)
> > 
> > It appears there is no user of counter_simple in this series other than the
> > selftest. Would you be planning to add any conversions in the series itself,
> > for illustration of use? Sorry if I missed a usage.
> > 
> > Also how do we guard against atomicity of counter_simple RMW operations? Is
> > the implication that it should be guarded using other synchronization to
> > prevent lost-update problem?
> > 
> > Some more comments:
> > 
> > 1.  atomic RMW operations that have a return value are fully ordered. Would
> >     you be adding support to counter_simple for such ordering as well, for
> >     consistency?
> 
> No -- there is no atomicity guarantee for counter_simple. I would prefer
> counter_simple not exist at all, specifically for this reason.

Yeah I am ok with it not existing, especially also as there are no examples
of its conversion/usage in the series.

> > 2. I felt counter_atomic and counter_atomic64 would be nice equivalents to
> >    the atomic and atomic64 naming currently used (i.e. dropping the '32').
> >    However that is just my opinion and I am ok with either naming.
> 
> I had asked that they be size-named to avoid any confusion (i.e. we're
> making a new API).

Works for me.

Cheers,

 - Joel