linux-kernel - Re: [PATCH RFC] Introduce atomic and per-cpu add-max and sub-min operations

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Date:	Mon, 15 Feb 2016 15:14:07 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>
Cc:	Konstantin Khlebnikov <koct9i@...il.com>,
	linux-arch@...r.kernel.org,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH RFC] Introduce atomic and per-cpu add-max and sub-min
 operations

On Mon, Feb 15, 2016 at 10:50:29AM +0000, Will Deacon wrote:
> Adding Peter and Paul,
> 
> On Sun, Feb 14, 2016 at 12:09:00PM +0300, Konstantin Khlebnikov wrote:
> > bool atomic_add_max(atomic_t *var, int add, int max);
> > bool atomic_sub_min(atomic_t *var, int sub, int min);
> 
> What are the memory-ordering requirements for these? Do you also want
> relaxed/acquire/release versions for the use-cases you outline?
> 
> One observation is that you provide no ordering guarantees if the
> comparison fails, which is fine if that's what you want, but we should
> probably write that down like we do for cmpxchg.
> 
> > bool this_cpu_add_max(var, add, max);
> > bool this_cpu_sub_min(var, sub, min);
> > 
> > They add/subtract only if result will be not bigger than max/lower that min.
> > Returns true if operation was done and false otherwise.
> > 
> > Inside they check that (add <= max - var) and (sub <= var - min). Signed
> > operations work if all possible values fits into range which length fits
> > into non-negative range of that type: 0..INT_MAX, INT_MIN+1..0, -1000..1000.
> > Unsigned operations work if value always in valid range: min <= var <= max.
> > Char and short automatically casts to int, they never overflows.
> > 
> > Patch adds the same for atomic_long_t, atomic64_t, local_t, local64_t.
> > And unsigned variants: atomic_u32_add_max atomic_u32_sub_min for atomic_t,
> > atomic_u64_add_max atomic_u64_sub_min for atomic64_t.
> > 
> > Patch comes with test which hopefully covers all possible cornercases,
> > see CONFIG_ATOMIC64_SELFTEST and CONFIG_PERCPU_TEST.
> > 
> > All this allows to build any kind of counter in several lines:
> 
> Do you have another patch converting people over to these new atomics?

The Changelog completely lacks a why. Why do we want this?