netdev - Re: [patch] ipvs: force read of atomic

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Date:	Wed, 08 Aug 2007 17:08:44 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Heiko Carstens <heiko.carstens@...ibm.com>, andi@...stfloor.org
CC:	David Miller <davem@...emloft.net>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	schwidefsky@...ibm.com, wensong@...ux-vs.org, horms@...ge.net.au
Subject: Re: [patch] ipvs: force read of atomic_t in while loop

Heiko Carstens wrote:
> On Wed, Aug 08, 2007 at 03:21:31AM -0700, David Miller wrote:
>> From: Heiko Carstens <heiko.carstens@...ibm.com>
>> Date: Wed, 8 Aug 2007 11:33:00 +0200
>>
>>> Just saw this while grepping for atomic_reads in a while loops.
>>> Maybe we should re-add the volatile to atomic_t. Not sure.
>> I think whatever the choice, it should be done consistently
>> on every architecture.
>>
>> It's just asking for trouble if your arch does it differently from
>> every other.
> 
> Well..currently it's i386/x86_64 and s390 which have no volatile
> in atomic_t. And yes, of course I agree it should be consistent
> across all architectures. But it isn't.

Based on recent discussion, it's pretty clear that there's a lot of 
confusion about this.  A lot of people (myself included, until I thought 
about it long and hard) will reasonably assume that calling 
atomic_read() will actually read the value from memory.  Leaving out the 
volatile declaration seems like a pessimization to me.  If you force 
people to use barrier() everywhere they're working with atomic_t, it 
will force re-reads of all the non-atomic data in use as well, which 
will cause more memory fetches of things that generally don't need 
barrier().  That and it's a bug waiting to happen.

Andi -- your thoughts on the matter?
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