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DateWed, 08 Aug 2007 17:08:44 -0400
FromChris Snook <>
SubjectRe: [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@de.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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