linux-kernel - Re: [PATCH -tip 00/12] locking/atomics: Add and use inc,dec calls for FETCH-OP flavors

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Message-ID: <20160624173028.GA24658@linux-80c1.suse>
Date:	Fri, 24 Jun 2016 10:30:28 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	peterz@...radead.org, mingo@...nel.org, davem@...emloft.net,
	cw00.choi@...sung.com, dougthompson@...ssion.com, bp@...en8.de,
	mchehab@....samsung.com, gregkh@...uxfoundation.org, pfg@....com,
	jikos@...nel.org, hans.verkuil@...co.com, awalls@...metrocast.net,
	dledford@...hat.com, sean.hefty@...el.com, kys@...rosoft.com,
	heiko.carstens@...ibm.com, sumit.semwal@...aro.org,
	schwidefsky@...ibm.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -tip 00/12] locking/atomics: Add and use inc,dec calls
 for FETCH-OP flavors

On Fri, 24 Jun 2016, James Bottomley wrote:

>On Mon, 2016-06-20 at 13:05 -0700, Davidlohr Bueso wrote:
>> Hi,
>>
>> The series is really straightforward and based on Peter's work that
>> introduces[1] the atomic_fetch_$op machinery. Only patch 1 implements
>> the actual atomic_fetch_{inc,dec} calls based on
>> atomic_fetch_{add,sub}.
>
>Could I just ask why?  atomic_inc_return(x) - 1 seems a reasonable
>thing to do to me.

For one restoring the old state like that can be racy and looses the notion
of atomicity. The new family of atomic_fetch_$ops also better express the
purpose of the call imo. Finally, the added machinery (considering it came
from fetch_op() NOHZ needs), was mainly suggested by Linus (although yes, we
don't have users for all the calls): https://lkml.org/lkml/2016/3/15/352.

Thanks,
Davidlohr