linux-kernel - Re: [PATCH] Fix the x86 specializations of atomic_[set|clear]

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Date:	Thu, 16 Oct 2014 20:19:08 -0400
From:	Patrick Palka <patrick@...cs.ath.cx>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Pranith Kumar <bobby.prani@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Hans-Christian Egtvedt <egtvedt@...fundet.no>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Fix the x86 specializations of atomic_[set|clear]_mask

On Thu, Oct 16, 2014 at 4:24 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Oct 16, 2014 at 12:49:43PM -0400, Patrick Palka wrote:
>> On Oct 16, 2014 4:02 AM, "Peter Zijlstra" <peterz@...radead.org> wrote:
>> >
>> > On Wed, Oct 15, 2014 at 04:38:01PM -0400, Patrick Palka wrote:
>> > > This patch fixes a number of issues with these specializations:
>> > >
>> > >   1. The memory operand inside the asm specification is erroneously
>> > >   declared read-only instead of read-write.
>> > >
>> > >   2. There is no reason to require the 1st operand of andl/orl to be
>> > >   inside a register; the 1st operand could also be an immediate operand.
>> > >   So change its specification from "r" to "ir".
>> > >
>> > >   3. Since addr is supposed to be an atomic_t *, the memory operand
>> > >   should be addr->counter and not *addr.
>> > >
>> > >   4. These specializations should be inline functions instead of macros.
>> > >
>> > >   5. Finally, the "memory" clobbers are unnecessary, so they should be
>> > >   removed.  (This is in line with the other atomic functions such as
>> > >   atomic_add and atomic_sub, the likes of which do not have a "memory"
>> > >   clobber.)
>> >
>> > No real problem with this, but I'm going to kill off these functions
>> > when I get a little time :)
>>
>> Hmm why's that?
>
> because they're odd (inconsistent with the rest of the atomic
> interfaces) and not implemented by all archs.
>
> See: https://lkml.org/lkml/2014/2/6/196
>
> 3.18 will include up to 4/5 of that series and when I get a spare moment
> I need cleanup/post the next arch sweep that will get us that 5/5 thing.
>
>

Cool! Perhaps atomic_inc_short() should be killed off too. It
currently has no callers and, despite what its name suggests, it is
not even atomic..
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